OK said Id do this so heres a QUICK kinda easy how I PERSONALLY trim armor. This is by no means the best or the most accurate way yada yada, and if you mess up your armor Please dont come knocking on my door, you take the risk trimming yourself.
So when you receive armor from a vendor it will be Vaccume formed plastic and with that there is flashing (extra material) that needs trimming.
What you want to do is use a dremel or other rotary tool of another brand at a medium speed not to slow and not to fast...take it slow. So find that speed that is just right.
I personally eyeball the thickness, Ive trimmed alot of armor and trust myself to make it even....I will sometimes go in with MANY MANY passes at a slightly lower speed to even it out and get it where i want it.
You want it to be about 1/8" thick looking when done. But again look at reference pics to see what your doing is right. You may think the armor should be 1/4" thick but this is not correct....it just kinda appears to be that thick by the marks in the armor the makers use on their bucks when forming the armor.
I use a few different bits for trimming helmets and armor...for the armor I really only use one bit the 1/2" diameter barrel, and I use exacto blades to cut out the chest lights...the others I use on the helmet to damage and trim it....the round looking diamond bit I LOVE for the ROTJ back left panel...it makes the same exact markings which is GOOD. The carbon cutoff wheel I use to trim the visor, you can use a hacksaw blade after slotting the area with the cutoffwheel for more control however it takes a bit longer....although it makes less dust.
Ive attached pics of before and after comparisons...these still need some sanding but basic idea is there.