Well, first you need to set the crankshaft to TDC by way of the harmonic balancer 30 | 20 | 10 | 0 | T | 10 | 20 the mark between the 10 and 0 is TDC.
Now set camshaft to TDC by aligning the mark on the lip of the cam (either a hole or a dot depending on MY -- I'll post a pic tommorrow, don't have my shop manual with me) with the dot on the rocker #1. If it's an inline six, cylinders one and six will be at their crest.
Now wrap the chain around the crank sprocket and set the right side of chain in the rail leading up to cam. Make sure the slack is coming off the left side of the sprocket into the tensioner rail, then install the chain tensioner to keep timing in place.
With both crank and cam at TDC, and tensioner ratcheted up enough for chain to not jump a link, turn crankshaft with 27mm socket until back at TDC. Compare the two .... as long as they're within a couple of degrees after the two cranks you're ok. If dead on even better.
This advice applies to a 103 engine and I'd imagine the 104 too along with any other inline 6. 5cyl diesels and V8's I have no clue.
'91 300E, 212K miles