NOT a good method ! Do NOT do any soldering on the MB.
Instead, cut the wires off the old battery as close as you can to the battery. Trim a little bit of insulation off the wires. Solder those wires to the new ones. (This also makes it easy to get the colors right.) If you don’t have heat shrink tubing or such, peel off some yellow cellophane tape off the old battery and wrap each splice.
Be VERY careful no to short any leads while doing this. Keep each lead on the new battery covered until you’re ready to splice, solder and re-cover.
This brings the difficulty down a level.