Guess there are a couple of questions you'll have to ask yourself, friend:
1) How much do you know about leatherwork at the moment?
2) What tools do you have at your disposal?
3) How involved do you want this to be?
4) How much are you willing to spend, which relates to:
5) How much leather-work do you plan on doing in the future?
That being said, I've made bracers, sleeve garters, cuffs, and all sorts of similar things, so here's what I'd do:
1) Get on ebay or something similar, find someone selling leather scraps at least 1' square in a vegetable tan (veg tan) leather at somewhere between 8 and 12 oz. Buying scrap-leather in bulk at an appropriate minimum size and weight is far more cost-effective than retail.
2) Measure your wrists and forearms at the points where you want the cuffs to start and end.
3) Make patterns out of stiff paper until you find the proper shape...they'll be elongated trapezoids with semi-circular top and bottom cuts for simple cuffs, but if you want the tapered look of some of the more elaborate examples you posted, the shape with be more complicated, accordingly.
4) When your pattern is perfect (considering the method of attachment, whether they'll have a buckle-closure, lacing-closure, etc.), trace the patterns onto the leather with a stylus of some sort, then cut.
From here it's about details.
You could tool the leather, for which you might want to procure tooling tools, though it can be done with anything hard and textured and a stylus of some sort, provided you have warm water and a wood or rubber mallet.
You could create a lacings-closure with just a leather hole punch and some shoelaces or leather lacing, though it might be better to buy a leather eyelet kit.
You could attach straps and buckles for a buckle-closure, but for that you'd need a slightly thinner leather, a strap-cutter, rivets, and buckles, though none of those things are too difficult to obtain.
You could roll some MUCH thinner leather over the edges...you'd want to water-shape them with warm water and either stitch them on (which would require leather glue, leather repair glue, or something similar first, then an awl or tiny leather hole punch, and appropriate cordage) or rivet them on (again, probably best to glue first, then punch the holes, then rivet). This would essentially 'edge' the cuffs.
You could line the cuffs with some softer material on the inside, which would again require a proper glue, and stitching, and probably edging afterwards.
You could adorn them with various plating, which would require probably thinner brass or copper plating, something the bend or shape them, then rivets. You could also just drill holes in medallions or coins or what-have-you and rivet them on as well.
You could stain the leather or polish it with various leather dyes and shoe-polishes.
You could take some thinner leather and rivet or stitch them on for the bullets in that last picture, or whatever else you'd want on there, like vials of various sorts or whatever.
You could build in sheaths, interior or exterior, for knives, shuriken, letter openers, whatever.
You could mount in a pocket-watch, a compass, whatever.
You could water-harden (boil) or wax-harden the leather to armor-like rigidity, but you'd want to initially build the cuffs larger than you want, as the hardening processes typically cause the leather to shrink some.
You could rivet on studs, buy screw-in spikes, etc.
...anyway, the world is your oyster. Start with the basics, I suppose, and don't hesitate to contact me if you need to troubleshoot something.