The Armour Archive

is there any secret to it? do i need to add on the sides, top and bottom for dishing?

how about the sides, do i make the lines vertical or make them have a bow outwards to allow for dishing?

i've never made a pattern before and thought i'd try it out

One approach is to take your best guess, maybe try it out with manila folders first to make sure it's in the right ball park, and just pound it out in the cheapest steel you have to see if you're about right. Adjust it for your next run.
My own experience hasn't shown a huge need to adjust dramatically for dishing "sucking in" metal from the edges, since the metal is generally stretching out some. It does change the form though, particularly with a deep dish. My globose breastplate pattern looks a bit like an upside down Y rather than the T shape you might expect, since the dishing is deep, and I rotate the top toward the collar bones to get the belly where I want it.
Figure out the general width and height you'd like, and add on extra for rolled edges if that's your plan. You can always grind more off if need be later, though it can be awkward.