I can't cover a wide range of leatherworking info in 20 minutes, so i'm going to focus on two topics that are most likely to come up in craft builds--making straps and edging things with a leather binding. Because i've not taught this particular topic before, i thought i'd write up my info in a Tutorial post here as a way of practicing it and getting my thoughts in order.
These are the tools I recommend that you have on-hand for working with leather.
On the left is a metallic gel pen. You can use these to write on the "right" side of leather, and the ink wipes right off with a bit of soap and water (or spit, even). Marking on the right side will help you avoid any scars or blemishes on the hide when you lay out your pattern pieces or measure off your strap lengths.
In the center is a strap cutter. This is a device for cutting heavier-weight leather, usually for beefy belts and baldrics. The cross-piece has a blade on one end and a ruler marked along its length. You slide the handle to the point on the ruler that equals the desired width-measurement for your straps, then snug down the bolt on the end. You feed the leather into the gap in the crossbar and pull it through, with the blade cutting off your strap for you. This tool requres one side of your hide already be cut with a straight edge. (Be careful when you get to the end of the leather--it's all too easy for the strap cutter to fly off the end and if your free hand is in the way...well, let's just say that lodging a strap cutter blade in your hand is about as much fun as stomach flu.)
You will also need a rotary cutter (on the right) and cutting mat (the big green surface), and a steel ruler. They make it easy to cut long straight straps and bindings from thinner leathers.
Straps are needed SO frequently in craft projects--for belts, baldrics, bag-straps and handles, wrist-loops for parasols or fans, chinstraps on helmets, buckle-straps for pieces of armor...the list goes on. Unfortunately, many costumers haven't had much experience with strap-making, and otherwise-beautiful craftwork winds up needing frequent repair due to wimpy, poorly-constructed strapwork.
With theatrical costuming, the primary consideration for all straps is durability and longevity. Think about it--an actor whose costume involves armor will buckle and unbuckle his strapwork dozens, perhaps hundreds of times in the course of a run, the average piece of armor may get reused in many more productions after it is built. How much wear-and-tear do those load-bearing straps take in their usefulness-lifetimes? Tons.
Frequently, you see single-layer straps in costume work: a strip of leather with a buckle riveted on, holes punched in, etc. This is the least-durable strap you can make. If at all possible, avoid the single-layer strap. You may have to use a single-layer strap in some situations--the designer insists, or you don't have enough leather or budget to buy more. If you must do so, try to attach them with Chicago screws rather than rivets, so that it will be easy for you or some other future craftsperson to replace them when needed.
In Fig. 2 (below), you can see three different ways of making multiple-layered straps: the two-layer strap, the two-layer edge-bound strap, and the three-layer strap. At the bottom is an illustration of how to create a finished end-tab on a 3-layer strap.
The simple two-layer strap is easiest to generate. I find the quickest way to make these is to adhere two large pieces of leather together with Barge Cement (use your respirator, as Barge is a carcinogen!), measure out the straps and mark them with a gel pen, stitch them about 1/16" to either side of the pen lines, and then cut them all apart with a rotary cutter. I like to use two-layer straps when i need something that looks delicate but is durable (like purse straps), or when i need a narrow width for a small buckle (i.e., helmet chinstraps or women's shoe straps). And get this: a two-layer strap constructed in this fashion of two pieces of 2oz leather is stronger and more durable than a single-layer strap cut from 4oz leather!
The two-layer edge-bound strap is good for a number of things--slightly fancier-looking belts and baldrics, for example. This construction can "dress up" a coarse belting leather by using a nicer, more supple leather for the backing/binding layer. You can also create a visual contrast in the strap with only one type of leather with this method, by using the finished side for one layer and the suedey back side for the other (as in the photo above).
In creating this strap, be sure to cut your backing/binding layer wide enough to fold over on both sides, with a little bit of ease (like 1/8"-ish) added in for the actual physical folding part of the program. I slather both layers in Barge, then after adhering them together and folding over the edge-binding, I run them through a walking-foot machine, stitching down the binding through all layers for added stability.
The three-layer strap is the strongest construction method for a "smooth"-looking strap. It is also a great way to make a durable strap from thin "wimpy" leather. It has been my experience that costume designers will purchase leather that looks the way they want, whether it is the right thickness for what they want done with it or not. The three-layer strap is a great way to take, say, a hide of buttery-soft 2oz metallic-foiled pigsuede and, by using a heavier weight leather for the backing layer, create some big strong three-layer armor straps that will be both durable and aesthetically acceptable to your designer.
To make the three-layer strap, i glue and fold over the top layer, pressing it down nice and smooth with a breyer or a rolling pin. Then i glue the backing layer on and stitch along both sides with a walking-foot machine.
At the bottom of Fig. 2, you can see a diagram for how to cut and fold the end of a strap for a nice finished triple-layer point. Follow the diagram on the left for the strongest construction. If you do your endpoint like the diagram on the right, it will be weaker because the layers will abut right across the tip of your strap; this will make the pointed tip want to curl up away from the strap. Not ideal. And, if you will be making a lot of three-layer straps of the same width with finished pointed ends, it's easiest if you make a template of the left-hand cutting diagram out of thick cardstock, and then just trace off your cutting lines for those notches.
Leather binding makes a nice finish for any number of items--hat brims, leather or metal armor pieces, bag-flaps, etc.--and can do double-duty as a safety element (eliminating the wrist and ankle "bite" of steel bracers and greves) or aid in actor comfort (as with a soft pigsuede binding on a hard leather belt). Above you can see a curved edge binding and a mitered-corner binding.
When binding a curved edge, i like to remember this schoolyard joke:
Q. Which stretches more, skin or rubber?
A. Skin. Moses tied his ass to a tree and walked 40 miles.
Har har, i know, but it does still make me laugh. And, it's an important point: leather is amazingly elastic.
On the curved binding above, note that there are no notches cut out. Think of leather like bias tape--it can take a pretty serious curve if you are careful to smooth out the edges and pull it as you apply it. As with strap-making, i like to glue first, then stitch when i put on leather edge bindings.
When turning a corner on a leather binding, remember: fewer layers involved makes for superior construction. Layers = bulk, and bulk is your enemy. It makes little nubby nodular points on the costume piece that could chafe the actor, and the more layers you have, the less your machine likes to stitch through them (ergo, the more likely you are to break a needle, cuss a lot, and maybe even put your eye out when broken-needle-bits go flying like errant clipped toenails). So, miter your corners! You can do it without cutting your binding in advance--glue it on, and when you get to the corner, smooth both straight-edges down right into the corner, so you wind up with a little triangle of your extra binding sticking up at an angle like a sundial gnomon. Use some sharp scissors to snip that extra off, and there you go: a mitered corner. I always then reinforce the whole thing with stitching, of course.
So, there you go: the fool-proof high-quality La Bricoleuse-approved method for making leather straps and edge-bindings. I'm not saying these are the "right" ways and all other ways are wrong. I'm just saying, these are the methods I like to use, and my straps are durable, strong, and designers think they look like hot shit and cold beer. Er, by that i mean, fantastic! :D