Hello and welcome to Just Answer,
It is best to have a manual. I do not have the manual or I would post it for you. Here it is in a nutshell.
The first step is to open the small cover at the front of the baler so you can see the plunger arm. Rotate the flywheel until the plunger are is at top dead center and between the two timing marks etched into the baler frame next to the plunger arm. Then make sure the needles are in the home postion. One other important thing is the needle yolk rod. It is located on the right side of the bale chamber next to the twine box. Grab hold of this rod down where it connects to the needle yoke and give it a pull towards the rear of the baler. It should come back a little. If you do not do this, it will be out of time. The next step is to check and see if the timing marks line up. If not, then remove the chain and move the timing mark, re-install chain, tighten chain, and check timing. Sometimes tightening the chain can change timing enough to engage the needle latch so always check timing after tightening the chain. Adjust as necessary, trip the knotters and run the whole machine through a cylcle by hand.