Pre-tin one pad, solder the battery holder onto that. Afterwards solder the second leg of the holder.

just take your time, pre tin one pad, use pliers or tweezers to push a part into the hot solder. Then solder on every other leg or pin of the part.

Add solder to one pad, use pliers to hold the hin in place and solder everything together. After that use heat the second pad and the clip simultaneously and add solder to fix up the second pad. You need to start with the clip because it takes a lot of heat. It makes sense to let the board cool down afterwards.