took my topend apart down to the head gasket, to change gaskets.

when we were about to pull the head i turned the engine to TDC and put a mark on the cam chain and cam gear   that's not how you do it

went to pull the head and my buddy dropped slack on the cam chain and lost position

so how do i reassemble with the cam perfectly insync with the crank??

The chain has no understanding of where it lines up on the gears, the cam is marked, hard to see, but marked, those lines lay horizontal, lined up with the top of the head./ Set the piston at TDC, lay the lumps down on the cam, line up the lines, put the chain on,,
LOOK Carefully at those lines on the cam & look Carefully ( Get your eyeball Down Level with the marks inside the left cover port.,) & make certain its still at tdc.