I've heard of people managing to get the outer fender installed after first installing the inner fender, but when I did mine a few years ago, I installed the inner fenders and outer fenders as assemblies, loosely bolted in place first, then mated the lower valence and upper header panel - got the gaps where I wanted them and snugged everything down.
Due to the way they go togetheer and my very limited experience working with sheet metal, it seemed to make a lot of sense at the time to install the inner and outer fenders as an assembly. Worked out well.
Good luck
Richard