It's a bit difficult to give an exact mix ratio H, as I make it to the requirements of the job. Basically, I pour some Future into a small container, such as a yoghurt pot (ensuring all the yoghurt is gone and the pot is clean of course!), then add talc until I get a sloppy goop, rather like porridge, hence my name for it.
I use Future for a relatively thin, 'fine' mix, for very small gaps or depressions, and enamel gloss varnish for a slightly 'heavier' mix for small, but not huge gaps.
Once you've established what mix is required, it's run into the gap and any excess wiped off. Allow to harden, which is much quicker with Future, and repeat as or if required. It should find its own level, and normally doesn't need any sanding.
For very tiny gaps, such as the joint between the one-piece spine and the top of the main fuselage on my MiG, and similar tiny, almost invisible wing joints, then PVA can be run into the joint with the tip of a cocktail stick, and the excess wiped off immediately with a wet finger tip. Again, repeat if required. Once this has hardened, anywhere between 15 minutes and 60 minutes, depending on density/area, it'll be clear, and can be painted over.
Hope this helps a bit.