Bleaching just the regrowth / roots

This is something I've never done.  😮  is there a beginners guide somewhere?

When I had long hair, I had a dip-dye thing going on.  With my undercut, by the time I had significant regrowth it was time to shave it off.
Now I have sort of pixie / short-back-and-sides hair I'm confused!  So far I've let it grow out, get it chopped, start again.....  but that gives me an awful long time of it looking rubbish. Well I get two phases out of it; phase one is all-over colour, phase two is natural on the back & sides and some colour left on top but with natural roots.

I'd rather re-do the colour all over when I get it cut (or even just on the top to wipe out the dark roots) but that would require careful sectioning and application of bleach on a semi-regular basis  😮  how the heck do you do this on shortish hair?

I'm currently a dark blue (that was meant to be purple!!) with my natural dark brown regrowth, so it's not terrible but I'm not happy with it.  Luckily it's nearly hat season  😉

Oh, also, is it true that you shouldn't bleach your hair while pregnant?  I can't find anything definitive.