JSFM35X !
Hope all is good with you and your family !
Coming from the absolute World of Convertibles - Seattle, WA., and of course Detailing several hundred up there, my best method was using this product -
https://www.autopia-carcare.com/raggtopp-fabric-top-kit.html#.W0_ttdXFj3g
After pressure washing it, then immediately spraying the cleaner, then brushing it carefully into the fabric, and finishing it with the Mytee Extractor, what water was left was removed with the Master Blaster..
This RaggTopp Fabric Protector Spray is very easy to use and control.. Just a couple of coats down, and the Top is beautifully dark again, and repels water like crazy for a long time..
I never really sprayed the product much on the vehicle, but if I did, it was very easy to remove..
What you can try if you want to, is cut one of the the folding ends of a really long cardboard box and use that long rectangle as your Edge right under the top, where it meets the rest of the vehicle.. The wider, the cardboard, the better, but not too wide either.. You have to be able to control it with one hand.. Hold it tightly on that edge, spray to the edge, move the cardboard to the next long spot and repeat..
I never tape up, put down plastic, etc., nothing.. No time for all that..
It`s an old House Painters trick I learned over 40 years ago, when I decided I needed to paint houses for extra cash AND all the other things young, fearless, stronger than heck men did back then..

I did this to spray the trim color and under the eaves if that was a different color, and not get any on the just painted body color of the house..
They even sell a bracket on a short pole that holds cardboards like this (and the cardboards) in the Paint Section of Home Depot...
Ever wonder how a painter sprays that perfect edge from the white ceiling paint to meet the different color paint of the walls up there ??? Yep, this same cardboard edge..
Dan F