Actually flashing has quite a lot of variables that you can use to your benefit, especially with multicontrast paper. To dispel one myth posted earlier, pre or post flash does make a difference, just as if film were slightly (maybe heat fogged) and you then took pics with it. The material (film or paper...and I'm going to assume multicontrast here just to make things easier) is already sensitized if it has any exposure to it before exposure, and therefore whatever that 2nd exposure is will build upon the first (the pre-flash). Going back to paper, determining just how much to use before your main exposure is a trial and error, since there are so many variables in the process...from contrast filter used (yes it DOES make a difference in your pre or post flash), to contrast range of your neg, to the type of enlarger light source you are using, etc. Personally I've always used an enlarger to flash, since I can control the amount of light with both the aperture of the lens and the timer. You can also make a mask to go between your lens and paper to mask off areas you don't want to get flashed (I put a paper box on the easle, then cardboard, refocus the image on the cardboard, trace the image, cut out what you want flashed, refocus the enlarger and try and hold the cut out at the same height you traced it from and you will get a darn good mask). Since you mentioned sky, I'd flash with a 00 filter first, the burn with a combination of filters to bring out the sky...maybe a 1 and 3F combo to bring in some of the lower end density, then separate that with the 3 filter, and maybe a post flash but it really depends on your neg so its hard to say.
It takes a lot of trial and error, paying attention to your results, and practice but it works, and very well. Hope this helps