Naast clan will definitely steer you right
. Looks like you're off to a good start there. For attaching the knees and shins, what I usually do is sew some velcro onto the pants/flight-suit-legs, a single strip right down the middle of the knee and shin. That'll keep them 'on', while straps on the plates themselves keep 'em snug and stop them wobbling around.
What you can do is get some 1/2" elastic for the knees and either attach snaps or sew velcro on. I like to go with velcro; attach D-rings to the plates and make it so you can pulled the strap through the D-ring then velcro it to itself. If that doesn't make sense, I can see if I can rustle up some pics of my knee plates to show it. For the shins, your best bet is 1" nylon strapping, with some elastic sewn to the part where the straps attach to the plates to give you a little flex for comfort.