Way I did it, step by step so I know I've not made any assumptions and left you confused.
Get into your flightsuit/vest.
Put the velcro together so that the unattached sewy side is hooked onto the glued in place side on the plates.
Using a tiny amount of paint dab some on the corners of the velcro
Line up plate with where you want it and press against flightsuit/vest for a moment
Move plate, check the paint has transferred to the vest/flightsuit.
Take everything off and sew that bit into place by matching the velcro to the paint.
Repeat for next one.
If you're doing the leg armour, wear whatever you have, girth/belt/boots etc as they can affect where the flightsuit sits.
If you're doing the vest start at the collar and work downwards one plate at a time so that you can use the plates above to help get the spacing right.