If you don't know how to do this it may seem kind of intimidating, but in fact it is extremely easy. The CB650 (probably along with every other CB bike of the era) has hardened steel valve seats pressed into the head. The purpose of lapping is to slightly grind off some valve seat material until the valve and the seat make an air-tight fit. You'll need valve lapping (sometimes called grinding) compound and a valve lapper, which is basically a wooden dowel with a suction cup on the end.

The first step is to place a small dot of the compound on the valve seat. I smear it on four spots as pictured on the left side.

Making sure there is no dirt in the valve guide or on the valve stem, the next step is to simply place the valve in the head. It is also a good idea to give the valve stem a light coat of engine oil beforehand.

Once the valve is in the head, the next step is to wet the suction cup on the lapper and stick it on the valve. Once the lapper is grabing the valve, you begin the grinding process. What you want to do here is place the lapper inbetween both hands, and using light pressure rotate the tool back and forth at a moderate pace. It kind of mimicks how you would go about starting a fire with a stick. Every couple of seconds lift the valve and rotate it 180 degrees to ensure the grinding compound is evenly spread.

Periodically you'll want to lift the valve out and see the progress. What you want is a consistant gray ring on the valve seat - that is there should be no breaks or high spots.

You will see the same consistant gray line on the valve. It is not necessisary or good to over lap the valves. It is possible to remove too much, and doing so will ruin the valve seat. (Don't worry about it though, because you'd really have to be going at it to ruin the seat) On the other hand, if you are unable to achieve a consistant gray ring the seats are probably out of spec and require professional cutting. Make sure you remove all the compound, it is highly abrasive and will make your engine very unhappy if left in.