I keep it simple. For buttons I roll a tiny ball of Magic Sculp, then press it flat. Letting it set up a bit so that its firm works best. I just use any sharp,round object to make the holes in the button, if the scale is larger and they could be seen. For straps I roll out MS, let it firm up, then cut it into appropriate width strips, then add them to the fig. Its a fine line between too firm to work with, and not firm enough. Sometime I will just sculpt them on the figures by rolling out some MS into snakes, flattening it where it needs to be on the fig, trimming it with an xacto, then smoothing it out. Repeat as necessary. If it need to be raised off the surface, I just wait until the MS is very firm, and lift it slightly with an xacto blade.
For belt buckles, I put a firm blob of MS where it need to be on the belt, flatten it out and smooth it, then cut it to shape. Then I use a rounded X-acto knife blade to cut in the inside border/belt part. If I need to, I will wait until that dries, then go back in and add the belt, slight raised to go up over the keeper, push in a couple of holes, and add the prong through one of those holes.
This is all from working in 1/16th scale or larger, my eyes aren't good enough to do anything much smaller any more.
Cheers
Jason
EDIT
Holy crap, I just looked at Gordy's method for buckles, and we do it basically the same way. That can't be right, I don't do anything the right way!!!!