Hello and welcome aboard, here's my way of getting straight lines:
First I like to mark trim lines with a pencil, easy to rub off again.
Being ATA it is a little thicker ABS than most other armor so I like to use a rotary tool with a cutting disk to roughly trim. Time for the first cut
For those pieces having cover strips added, arms/biceps 15mm, thighs/shin fronts 20mm and backs 25mm, divide by 2 and you have how far you need to trim from the outside ridge.
Here is the first thigh so 1/2 of 20mm is 10mm, so measure from the curve section ends on the top of the ridge by 10mm. I like to use a steel ruler and clamps to get a nice straight line.
As this is a build for Jerry I'm going to leave some extra material on the backs of pieces for a test fitting, I prefer to have the fronts a standard size and adjust the rears if needed, you can always trim off but it's hard to add on .
I like to use a sanding drum attachment to get excess ABS off.
Also go around all the edges with some fine sandpaper to get rid of any sharp edges, you don't want "armor bites"
Then to get those lines really straight a long sanding block.
Normally I would trim everything in one go but due to some issues with my hands I find it's easier to take a break from trimming and do some assembly.
On to assembling the thighs, I use some painters tape and clamps to fit the pieces together to make sure they align well top and bottom.
Then tape across the centres to pull them together.
You can then lay a piece of tape down the length of the inside of the thigh.
Pull off the outside tape and make sure you rough up the pieces with some sandpaper where the cover strips will fit.
Time to make some cover strips. I like to use one long ruler and another to measure widths, mark with a pencil, then score down 3 or 4 times, you can then snap the piece off.
Lay the cover strip on top of thigh and mark then trim length and round the corners to take away the sharp edge.
When you are happy you can then add some E6000 glue down the middle of the cover strip, spread with some scrap ABS and with clamps and magnets apply to thigh. I also like to use painters tape on any sections that don't pull down quite enough.
As I said earlier I won't be gluing the backs until we have had a trial fit.
Not going to bore you will assembling every single piece, besides when I'm on a roll I tend to forget to take photos
Continue with the other thigh, then shins, biceps and then forearms. Just make sure you have the correct width cuts and matching cover strips.