Good advice from Roy. It's easier to do after seeing it done rather than reading about it.
I have an old 10 inch jointer with straight knives and it's a PITA to get set right. I use a pair of magnetic guides. They hold each blade and sit on the cutter head. It should be easy, but my old machine has nuts behind each knife and they slip a bit which makes perfect alignment tough to achieve.
I'll be replacing it with a 12 inch with a spiral cutter head as soon as I can.
One thing you can try is two good rulers resting on the outfeed table,one on each side. The knife should just barely make contact with the ruler as it comes around. Where the knives top out as they rotate has to be parallel to and dead level with the outfeed table.
Dave.