Clean both the inside and outside of the respective pipe and fittings. Put flux on, push the pieces together in the proper orientation, heat the fitting until the flux is obviously hot, remove the heat momentarily and see if it will melt the solder. If not, continue heating. If it is a big fitting, you might need to heat one side while putting solder on the other after moving the torch around the fitting. It will suck the solder up into the joint. If you haven't already, remove the heat. WIth a cloth that won't melt, wipe the excess solder off before it starts to solidify and don't move the pieces until it has set up. Most fluxes are acidic, and any excess should be wiped off. The extra solder isn't a big deal, but it looks better if you wipe it off.