It is a bit complicated, and a shop manual is really the best source of the entire procedure to assemble the cams complete with the pictures. You really should have the manual for this type of work. The following gives the basics of timing the valves.
The T mark on the crankshaft is Top dead centre . Both pistons will be at the top of their stroke when the T mark lines up with the index.
There are 2 circular punch marks on the left side of the cam sprocket. When the cylinders are at top dead center a straight line between the 2 punch marks on the sprocket would be parallel to the top of the head. Rotate the right side of the cam so the sprocket bolt holes line up when the sprocket is assembled to it.
There is a notch in the end of the left side of the cam. The notch should point horizontal to the rear of the head, and the bolt holes should line up through the sprocket. The 2 punch marks should still be horizontal and the crank should still be on the T mark. Fit the first sprocket bolt and then rotate the engine so you can fit the second one. Torque them to spec.
Rotate the crank counterclockwise throught several revolutions and chack your timing marks again.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville