You start with the pattern. I use old file folders for that purpose...they are cheap, simple, and always available. I simply measured how long and how wide the knees should be, and since I use a combination of raising and dishing, the finished size is identical to the pattern dimentions. Below, you can see the pattern, the cut out knee cop blank, the tuck pointing tool and the results of the first go 'round with the tuck pointing tool. Below you can see the tuck pointing tool mounted in the vice. The idea is to make flutes in the metal.
As you can see in the below picture the flutes are starting to show up. This is after a little bit of judicious bashing with the hammer into the dent in the wooden anvil.
And so it begins.
There are a few scars on the outside face of the knee cop, most of which should sand out later. I hope...grin!
Tuck pointing is a bit tricky to do...it is technically a shrinking method. The idea being to shrink the metal rather than ballooning it out. The gauge of steel would increase when you tuck point, and decrease when you are just bashing the metal down into a dent in the wood. Tuck pointing is the normal first cannon used when you want to "raise" metal. I tend to do a bit of both, which results in better dimentional stability, and unchanging gauge sizes. Raising is preferred in period armour making since heavier metal allows you to have extra metal available when sanding off hammer marks. I don't care much about that since I use an english wheel to get rid of hammer marks.