Over the years I have made many washers by cutting a new leather disc, mounting it on the piston and using a jubilee clip to form the cup.
But the other day I wondered if there was an easier way, especially if you expect to be making a few washers. The answer is 'yes':
I just got a bit of delrin and cut a flat bottom hole about 7mm deep in the end to match the od of the washer. This is the'former'. Then cut a short delrin rod to match the id of the washer; I used the od less 4mm for 2mm thick leather although I have found this size also works for 3mm leather. This is the 'pusher'. A 6mm hole through the centre of each part and a 5cm long 6mm bolt and washers completed the equipment. I used a mini lathe to make the delrin bits but they could just as easily be formed from wood / doweling (broom handle?) using hand tools.
A leather disc a couple of mm in diameter larger than the total diameter of the washer, say the od of the formed washer plus 10mm to allow for forming the sides, with a 6mm hole in the centre completes the kit. Soak the washer in leather softener fluid ... shoe stretcher fluid is iso-propyl alcohol.... assemble it over the delrin / timber former with the 6mm bolt and wind the pusher into the former, so pushing / squeezing the leather into a good flat bottomed cup. I did the pushing operation gradually but you will soon have a good shaped cup. Remove formed washer and soak in suitable oil overnight ... I use neatsfoot oil ... then return to the 'press', tighten up and leave a few hours. The leather will no doubt stretch a bit and end up with sides a little deeper than required so simply trim them with a very sharp blade while still in the former. But I advise trimming to leave a little extra for fine adjustment once the new washer is finally fitted complete with the central fitment on the piston end.
Sizes can, of course, be adapted to cope with different od sizes and different leather thicknesses. So far I have used 2mm and 3mm thick ... the 3mm thickness was made on the same former as the 2mm and pushed easily into place. I have not yet used the new washers but they look excellent and are pre-lubricated.

I really must get out more rather than fiddling like this, but at least my wife knows where I am and I can't get lost.

Cheers, Phil