Assuming it's plexiglass, this is what Ive done for long straight cuts
I usually tape off the straight line on the "good side" jsut to make sure that when Im cutting it stays straight. I've got 2 long metal straight edges, so I keep one right along the taped edge and repeatedly score it with either a razor blade widget thingy, or Ive got a regular plexi cutting blade(usualloy hanging right next to where you find the plexi) When it seems deep enough I square up the cutline right over the edge of a table or bench, still applying pressure on the straight edge, I then take the other one which is really a beam level(so its nice n sturdy) butt it right up to the edge of the other SE, at this point its hanging over the table and resting on the "scrap" piece. Give it a good WHACK with a rubber mallet/coated ballpeen hammer right in the center of the level and it just snaps nice n clean. This works good on thin cuts- plexi usually comes in 24" width, but the size I need is about 23-1/2" SO theres not a lot of scrap to get a good hold on
If small pieces are left attached to the good side, I use a good clean pair of pliers, line it right up to where the cut should be and just apply a little downward pressure. 99% of the time it follows the score line and leaves it nice n smooth