RIGHT
Prepare to be assaulted with pictures.
I've been needing a new vest for some time and actually got the leather to make it a while ago. I used some lovely kid leather, as said above two skins were needed. There's always scrap bits of course and there MAY have been a tiny tiny vest made from the leftovers.
I shall dispense with the pattern making bit because that's been covered already. Beyond that all I did was take my pattern, lay it on my leather, tape it down (NOT pin, it's leather. Pins are fine for woven fabrics.) and cut it out.
Another note about leather - I use *fairly* thin stuff, and my sewing machine is fairly heavy duty. It's not an industrial machine but it does have a cast aluminium frame. I WOULD NOT sew even thin leather with a machine that had a plastic frame, as for example many Brother machines do. The machine would flex and potentially break.
Because I forgot the all important first step of having a cup of tea, there's a couple of mistakes in here, and I shall point them out. They aren't BAD at all and you probably won't notice until I do point them out, but they are kinda annoying to me. Lesson learnt - always start with a cup of tea!
I started with the shoulder bells.

Hemmed the edges. This is just folding the edge over and stitching it down. Note when sewing leather, I usually start sewing in the middle of a seam rather than at one end - leather likes to stick to the surface of the machine, so being able to pull it through is important.

Half of one done...

One fully done. See the back edge is NOT hemmed - this is where it attaches to the vest.

Both done. You can see the pseudo - piping look almost starting to develop. I'd have to sew closer to the edge to get it proper.

I grab the front piece and work out the darts. I start by folding the bottom edge over by the appropriate amount, 2cm in this case for a 4cm dart. I use tape to hold the dart in place while I sew and I sew straight over the tape.

Scotch tape is INFINITELY better for this than normal tape, but I didn't have any scotsmen around at the time.

When pulling the tape off, it tears along the stitching line fairly well. You get a little of this -

Loops created by pulling. They don't go away but that's OK. They're rather minor in the grand scheme of things. Scotch tape comes up easier which is why I prefer it.
Both darts done. The left dart is a bit better than the right.

Next I join the front and back pieces on the right side. It doesn't matter if you start with the shoulder or the side seam. Make sure that you put the "good" sides together and sew on the inside.

It's not that clear but here's both of those seams done

Then I hem the right sleeve hole... which was my biggest annoyance...

Looks perfect doesn't it?

Until we get to this point. Where I go, DAMN. I was meant to attach the shoulder bell BEFORE hemming the sleeve hole.

You can see the two lines of stitching. It would have been a whole lot neater if I had done it the other way, and you would have only seen one line. And this meant that I had to deliberately make the same mistake when I got to the other shoulder.
I hemmed the bottom as well, all the way around the front and back.

And now it's starting to look like a vest.
I started the zips. Firstly the side zip. Note the orientation - the "top" of the zip is face down on the good side of the leather, and it's on the inside not the outside. This shall make sense in a moment.

I took apart the zip to make my job easier

I changed the foot on the machine over to the zipper foot and sewed on one side

And once that was done, I folded over the leather so that it juuuust came to the edge of the teeth and sewed it down again

That's how you make it neat and hidden!
I trimmed the zip to length

Did the same things to the other part of the zip on the other piece of leather and voila! A hidden zip.

The shoulder zip is always trickier, because you can't separate it. I start by lining up the shoulder seam, good sides together...

And ONLY sew the outside 3cm or so.

Then comes the hard part, seating the zip. You can see here how it has to go - good side of leather against top side of zip - but on the inside of the garment - and ouch my head. It works though.

And attached, though you can't really tell with black thread on black zip

Next to hide the zip. I start at one end - the extra length of the zip allows me to pull the garment through rather than having to start in the middle and then go back and turn around

Get halfway, 90 degree turn, go across the end

Finish off the U shape with another 90 degree turn. Hidden zip.

I trimmed it to length

And hemmed the sleeve hole and neck hole. See how the hemming makes it so the zipper can't fall out?

At about this point I managed to bend my needle. Listen to your machine folks, you don't want broken needles flying into your eyes. Then you won't be able to read my build threads!

Aaaaaand done.

It fits quite well, of course. It'll be good to get some plates on this thing!
There you go folks, any questions please hit me up. I will do my best to answer!