Some random thoughts. Not trying to insult your intelligence, I am just going to make zero assumptions. If you already did all this, then ust ignore me:
1. You pretty much HAVE to do both front and back cams to make sure it's correct, or else there is a 50/50 chance that the front and rear will be out of phase.
2. Remove the inspection plug on the magneto cover and the timing chain tensioners from both front and rear cylinders. Remove the metal cam chain cover things as well. Depending on the angle of the moon, sometimes you can get away with not undoing the camshaft housings, but it sure does make life easier if you do. IF you do, you're going to want a torque wrench, as bearings hate life if over torqued.
3. Align the F mark on the magneto with the arrow on the engine casing that you can see by looking through the mageto cover peephole.
4. The first mark that you need to align is on the exhaust cam of the front cylinder. The order really matters. The mark is stamped with a dash. This should be parallel with the top of the head and the '2' mark should be pointing straight out at 90 degrees from the head. Pull your chain tight up the outside of the head and over the exhaust cam.
5. You now need to point the '3' mark stamped in the INLET cam, straight up. You then place the chain over the sprockets so that there are 16 pins in between the '2' and the '3'. ..what does this mean? So both the 2 and the 3 point to a tooth. The teeth fit into the space BETWEEN the pins. So where the 2 points to, that is zero. From there, count 16 pins. On the space after the 16th pin, your three should point there.
6. Re-fit the front tensioner, so it doesn't jump a tooth as you turn the engine later.
7. Turn your engine anti-clockwise 285 degrees (almost one full turn) until you see the "R" mark come into view in the peep hole.
8. For the rear cylinder head you install the INTAKE cam first (different to the front head).
9. Same process, line the dash up with the top of the head and pointing toward the inlet manifold. Pull the chain tight and over the cam sprocket.
10. Same as before, the three points straight up and there needs to be 16 pins between the '2' and the '3'.
11. Tensioner in. Button it up and job done.