If your 1990 Ultra is the same as my 1993 Classic, it's not very simple.
[ul][*]You will need to remove your windshield...7 nuts and bolts.[/ul][ul][*]Lay a couple of towels on your fender.[/ul][ul][*]Remove lower bolts and loosen the other on both turn signals/spot lights, carefully lower to front fender.[/ul][ul][*]Remove the whole headlight assembly....bunch of screws and unplug bulb.[/ul][ul][*]Reach into the hole from the headlight assembly and you will need to back off 4 nuts that hold on the outer fairing to the mounting brackets. Now the outer fairing will come off with a little tug out and up if I remember right.[/ul]If nothing else, this photo will show you exactly what the brackets look like that the outer fairing is bolted to
Speakers are held in place with 3 or 4 screws apiece. I took a bunch of pictures as a how to for our older bikes, but never got around to documenting it. This list is from memory and should not be concidered exact procedure, but good enough if you take your time and even better yet have a manual on hand.
I may be able to e-mail better pictures if I have them. I am very happy with my J&M speakers. John is a sponser of this forum, check him out.