Depending on it's vintage, your gun may not have the "witness marks" to line up. If thats the case it will be strictly an experiment to find the exact place where the slide lines up. When found use ONLY finger pressure to rotate the barrel. There are legions of fine old colt 03s out there with pipe wrench marks on the barrel, put there by people who didn't understand that the slide HAD to be in exactly the right position. After removing the slide, the barrel is rotated back the same 180 degrees and then can be easily removed from the slide for cleaning.
Mark Lane to William Buckley: "Have you ever referred to Jessee Jackson as an ignoramus?"
Buckley: "If I didn't, I should have"