It will bulge out like an omelette, I would spray when finished, unless you see a sticky drip and want to harden it
I have found a good method is to cover surrounding surfaces with clingfilm, then nearby with masking tape and newspaper
If injecting through a drilled hole, you can stick masking tape over the hole, then puncture it for the nozzle, this protects the surface round the hole. It is possible to fasten a batten (wrapped in clingfilm) over the gap, and inject through holes, and then it will harden flat against the batten, which saves cutting back. If you do not wrap the batten in clingfilm, it will be very very difficult to remove afterwards.
As you have not used it before you wil probably put too much in, so you will have to cut it back with a breadknife, then sand smooth or use a filler before decorating. Or you could fix trim or architrave round the window to cover it. It must not be exposed to sunlight as this degrades it, but you can paint it with gloss for protection.
It will also escape round the back, to the outside of the house round the window or into the cavity.
I have found the Polycell foam to have a better texture than the cheap own-brand ones (they can form big gas bubbles inside which leave voids). you canb get it in the high stret.
Once you gave started a can, it will probably not be usable on a second day