Yellow has always been a bit of a cad to work with. Exactly how to do it depends a bit on what end result you want to achieve, but I always use a white undercoat for yellow miniatures. For a reallly dirty and gritty look like Bad Moonz I simply apply a heavy wash of snakebite leather/chaos black over sunburst yellow.
For my eldar I want a cleaner look and I suspect it might be something similar you're after. In the old days I would use and orange wash in the recesses, but I've since taken a fancy to using thinned down snakebite leather. I haven't had a go with the new Citadel inks yet (student budget means I have to prioritize food and beer over small pots of paint
) but I thik you could get very nice results with those.
Anyway, I digress: For clean yellow I'd go for white undercoat, sunburst yellow basecoat, maybe a thin yellow/orange wash followed by a wash of snakebite leather that I wipe off so there's only paint in the deepest recesses of the mini. Pick out larger areas with pure yellow and highlight with a couple of layers of white/yellow mix. Depending a bit on the type of miniature this could be done with either drybrushing or classic layering.
As for your other questions: I ususally base my minis first. Maily because I like to get a feeling of how they look on the table before I start painting.
I usually finish the bases last. Also mostly a force of habit as you could certainly argue that other methods would be easier.
I painted all my SMs based. And I rarely base paint white ... makes the over paint too light !! 
Hmmm ... 