I used a process called impressioning. It is simple in theory but somewhat devilish in practise at least it was for my lock. I bought a few brass key blanks on ebay and set about the task. First watching this excellent video about the process:
Jos Weyers is a world champion and he had a lot of good tips but I really had no luck at all to start with. The basic idea of impressioning is to insert the blank into the lock then crank it around to the left until its motion is stopped. It can't move any further because one of the pins will be preventing it, the pin that makes contact first (mechanical tolerance means one will contact first). You then jiggle the key up and down, unlike the other pins which move freely up and down the binding pin will be held by the pressure of the tensioned key. That pin will rub on the key leaving a mark (an impression). You repeat this turning the key to the right before jiggling and that will make another mark (as it is likely another pin sill rub). You remove the key and file the two marks away reducing the height of the key at that point slightly. With experience you do this just the right amount to take the key down to the next depth in the key/lock's official specifications.
Sounds easy but the marks are subtle, you need a fine file (mine was too course) and you need some magnification. I ended up with a tiny 45X pocket microscope bought on ebay for around £4 and I made do with my file. I wiggled, I jiggled but no marks. I take the key to work and look under a proper microscope, no marks!
After a few days I give it some thought. The lock is a decent one, a Yale and not the cheaper brass one but what looks like a steel one, at least it is plated. The key barely moves in the keyway, hmm. Out with the dremel, I remove material so that the key can move more freely up and down. I do this on all of the upper edges of the cuts on the side of the key. I try again and I get something!!! But the marks I see are nothing like those in the video above. The keyway of the Yale is very wiggly and when the blank is fresh only the side of the pins rest on the edge of the key. Instead of nice little dots I got a sort of bruising on the edges. Really the brass pins are burnishing the brass key slightly. After 1.5hrs the lock opened and it felt amazing! The hard part had really been assessing what was really a mark and what was not, my file is excellent quality but a little too course and I was forever trying to decide of something was a mark from the file or the lock. The keyway also made it difficult but mainly because I was not expecting marks like the ones I got.
I have a feeling that I chose the worst possible lock to begin learning but I learnt a heck of a lot!