Your planer has been putting out smooth, flat boards for quite awhile. You might have started noticing a few chips or dips in the surface. This means you'll need to remove the knives from the cutter head and sharpen them. That's the easy part. Replacing the knives in the cutter head and adjusting the cutting edges to give you that nice flat surface again is going to be tricky if you don't have a knife-setting jig. It's not impossible, but it will require attention to detail.
Unplug the planer's power cord. Operate the on/off switch two or three times to be sure you unplugged the right cord.
Remove the planer's dust cover to expose the cutting head.
Rotate the cutting head until the first knife is in the top dead centre position.
Loosen the jam-nuts on the first blade with an open-end wrench. Loosen the nuts only to the point where you are able to move the knife by hand. You will want to be able to quickly tighten the blade in position when you find the proper setting.
Attach the dial gauge bases to the cutting head. Place one gauge at each end of the cutting head, about 1 inch in from the end of the blade.
Adjust the height of the blade until you get the same reading on both machinist gauges. Take note of this reading. This will be the measurement you use for the remaining blades on the cutter head.
Tighten the jam-nuts and check the readings on the gauges again to make sure the blades did not move during the tightening process.
Repeat the process above for each of the remaining knives in the cutter head.
Variations in height of as much as .002 inches are acceptable for most applications. The closer you can get to exact, the better your results will be. It is a good idea to remove and clean all knives and hardware before setting and adjusting blades. Cleaning the grooves in the cutter head and lubricating jam-nut threads will make the process of adjusting the knives much easier.
Never adjust or perform maintenance on any power tool while it is plugged in. Planer knives are extremely sharp. Wear abrasion-resistant or cut-resistant gloves while handling these blades. A small slip can cause serious injury.