Setting jointer knives appears to be universally every woodworkers worst duty. When I talk to woodworkers, the one thing they hate doing most is setting jointer knives (although many also hated setting planer knives, but that`s another article).
One of the biggest problems is many did not know what height the knives should be set at. Many woodworkers believed that jointer knives should be set at the exact height of the outfeed table. Which in theory would be correct but in practice is not the best idea. I even had some woodworkers who adamantly believed the jointer knives should be even with the infeed table. This is the WORST thing you can do if you EVER want to get straight, flat boards.
If you set jointer knives BELOW THE LEVEL OF THE OUTFEED TABLE, the boards you run through your planer will be bowed like the lower runners of a rocking chair. The more you run them through the jointer the more bowed they will get ... to the point, the only way to correct the bowing is to mark a straight line down the length of the board and run the board through a table saw - freehand. THEN adjust your jointer knives correctly and start all over again runing the board through the jointer.
The proper height of the jointer blades should be just enough so that when you lay a straight edge steel rule across the outfeed table and the blade, when you rotate the blade backwards by hand, it will lift the steel rule ever so slightly, then move it backwards with the flow of the blade about one quarter to one eighth of an inch. Anything higher and you will get `snipe` that is too deep, anything lower and you risk getting bowed results, not straight flat results.
Of course the other thing to do is to MAKE SURE your fence is at 90 degrees to the the tables. The best way to check this is with a steel square. Any square that uses wood in one side is NOT accurate. I own one of these very nice looking squares with steel along one side and wood and brass along the other. The problem with the square is that depending on the moisture content in the air, the wood in the one side will absorb or surrender moisture, which means it affects whether or not the square is accurate or not .. SO, ALWAYS use a steel square to check your machine parts.
As a final tune-up for the Jointer, I like to use is Bostik Topcote on the tables. It is a spray that leaves the tables smooth, shiny and significantly reduces friction on the tables which makes it easier so slide wood through the jointer.
You can`t make quality woodwork projects if your tools are not set up properly and the first tool that MUST be set up is the jointer because all other tools receive wood that comes first of all from the this tool ...