Make the long lines short: Refer to "Disassembly Guides: Battery and Trackpad. Disassembling MacBook Pro 15" Retina 2012-2015" beneath the "Guides". Use thin string to remove battery cells as shown on the above mentioned "Disassembly Guides".

Macbook Pro Retina 15 mid 2012 battery replacement.

From more important to less.

1: Removing the main board was unnecessary to replace the battery. Just disconnect the power supply socket and two screws, then peel the battery cells out.

2: Be aware of flexicable from the trackpad on removing the cells. I did not care much about this and as a result I cut some lines on the cable.

3: Two cells on the centre are sticked onto the metal plate which covers the trackpad. That metal plate may be bent upon peeling the cell out. So be gentle.

4: There is one more connector beside the power supply socket. Pull that up when you really want to take the main board out.

I replaced the battery and found that the main board could not find the keyboard and trackpad, and the machine was extremely slowed down since "karnel task" used the cpu 1000% (not 100 but over 1000 on ActivityMonit). I'm not sure if I can fix it by replacing the trackpad. Anyway, I can not recommend yo to remove the main board when that is not really necessary.