Make sure the secondary mirror is centered in the focuser. Look through the focuser without an eyepiece and the secondary mirror should be centered in the focuser tube and look like a circle. If not, there should be a stud in the middle that will allow you to rotate the secondary and three screws that will let you tilt it until the image of the mirror is circular, don't worry about the mirror housing, just the image of the mirror.

Take it out and find a star about mag 2-3 and defocus it until you can see the shadow of the secondary with about 6 or 7 rings of light around it. The shadow must be in the center and you can move it with the three screws on the back of the primary. There might be 6 screws in the mirror cell which means one set is used for locking the collimation.

Turn one screw about 1/4 turn to start and note which way the shadow moved. Determine which screws you need to turn in order to get the shadow centered. Usually to tighten one screw, you may have to loosen the other two. Screws should be pretty tight in order to maintain collimation.

Nowadays they have laser collimators for Newtonian optics. I prefer not to use them but here is an example.