(Or not really but close enough. Also a side order of armour.)
I picked up this technique from this tutorial years ago, then adapted it further. You can use it to imitate leather, and with a few changes also to make thin armour parts, such as Vincent Valentine’s gauntlet as in the last picture. It won’t really work for thicker armour; there are plenty of tutorials on how to make EVA foam armour to be found online.
Back to fakey leather; you need the standard 2mm craft foam/fun foam/EVA foam/foam of many names, white glue, a brush, shoe polish, mod podge/podgy glue and a sponge.
You want to have most of the shaping and decorating done before you start. For example, for Athos’ pauldron I had already glued the three layers of the main piece and the fleur de lis shield together (UHU glue works well, all purpose glue is also okay), but had left the two upper strips and the straps separate to be attached after everything was done.
It is also easiest to do any embossing when the pieces are still flat. Foam won’t take an awful lot of detail, but with some patience the results can work quite well. You can use paper embossing tools for this, but make sure that the point isn’t too sharp or it will tear the foam. The back of a knife also works. My favourite tool for it so far is actually a letter opener. Drag across the foam for the first shallow line (it will spring back a bit) and repeat as many times as necessary without cutting the foam.
One layer of craft foam isn’t very strong by itself, so you either want to have at least two layers glued together or a single layer backed with fabric. If the final result includes stitching, definitely attach fabric or the thread may pull right through the foam. Cover the back of the foam piece with white glue, then smooth fabric onto it. Use cotton or linen (not synthetics), so that the glue can come through the fabric. When it is dry, cut the fabric along the edges of the foam, then brush another layer of white glue on the fabric. This makes sure that the fabric is firmly stuck and that the edges won’t fray.
(If you want to imitate metal armour, dilute the white glue a bit with water for the first couple of layers covering the whole piece; the fabric backing as well as the front and sides of the foam. This allows the glue to saturate the foam. Follow that with a few layers of undiluted glue, letting it dry in between layers until it’s stiff enough. The end result can be spray painted.)
Most importantly, when you glue layers of foam or foam and fabric together make sure that it dries in the shape it’s supposed to be. For Athos’ pauldron I wrapped a towel around a bottle to have something roughly the shape of my upper arm and then tied the pauldron around it while the glue between the layers was drying. Once it is dry, it will hold its shape. You do not need to heat the foam to do this! Heating it is inadvisable since foam is somewhat toxic. Foam is also a little stretchy, so you can carefully stretch it for an extra curve. Glueing fabric to it will help it hold its shape.
This is the point where I added battle damage to Athos’ pauldron by cutting grooves out of the top layer of foam with a stanley knife. The knife needs to be very sharp to prevent ragged edges, so use a new blade.
Now for actually making it look like leather! Rub several layers of shoe polish onto the foam, letting it dry between layers. The resulting colour depends on the colour of the foam and the polish. Use black foam and brown polish for dark brown ‘leather’, white or grey foam for lighter shades and so on (the Gondorian vambrace in the picture before last was made using white foam for the top layer and black foam for the bottom layer).
Then use a sponge to brush a layer of mod podge onto it. Mod podge is a glue varnish used for decoupage and is water resistant when it is dry. Two or three layers will do, and for a smooth finish dilute the mod podge with a little water for the last layer. Leave it to dry thoroughly. It will remain a little tacky, which can be solved by smoothing a little talcum powder onto the surface carefully. This also removes a bit of the shine, for a slightly more worn look.
Finally, assemble the piece, sew the bits that have to be sewn, paint decoration etc. As a general tip, if you want to make an object that looks like leather, treat it like leather! Add stitching, add metal grommets and so on. That goes a long way in making it look convincing.