For ls/na remove coil and pull first spark plug out. Ss has multiple coil packs.
Its a good idea to remove fuel pump fuse, start it, let it sputter and shut it off.
You need a compression tester. Has a gauge and hose with threaded bit. This hose goes in and screws into plug hole just slightly hand tight.
Have someone jump in and let it crank over a few times and watch the gauge. Usually like 6 cranks. Then disconnect hose from first cylinder and reinstall spark plug. Then move to the next.
For ls, 120+ is good. For ss, 180+ is good