RTFM is the best advice in this case but that shit can be hard to find here (and often it rubs me the wrong way) so here’s the rundown:
do you have room for a larger BMS in your board? If so you can use something like a DALY or a xiaoxiang smart bms. Wiring one of those into your pack would be easy, you just need to replace the old balance leads with the new ones that come with the new BMS and attach the new BMS to the negative side of the pack.
On those types of BMSs, the PCB will be labeled with a 0 - 12 on the balance lead connector for 12S and 0 will attach to the negative pole of the first group starting from the negative side of the pack (group 1). wire 1 would then go to the positive side of group 1, then wire 2 would go to the positive side of group 2, etc all the way to 12.
then you would connect the wire from the BMS labelled B- to the battery’s negative side and the C- wire to the charge port.
Because you don’t want your brakes to fail and the general sentiment here is “burn the board to save the life”, we generally don’t use the discharge protections from the BMS for the main outputs. It will interfere with the brakes so the BMS B- wire and the pack’s main output wire on the negative side will both be soldered to the pack in the same place, putting them in parallel, or a “discharge bypassed” state.
yes obviously shorting will cause bad things but don’t let these guys scare you off. I set plenty of things on fire in the beginning. Keep an extinguisher handy for everything around the pack, because you’re not going to put a pack out with an extinguisher. It’s for everything else. But more than likely the worst that will happen is you’ll melt a wire or blow a hole in a tool and the cells will be fine.
for more reading, check out this category:
and this thread in particular is full of good clean battery porn going a ways back