Lift and remove the battery.

Before installing your new battery, remove all the old adhesive from the MacBook Pro's case.

With a little luck, you can slowly pull out each strip of adhesive with your fingers.

Otherwise, soak each strip of adhesive with a bit of adhesive remover for 2-3 minutes, and then scrape it out with a plastic tool. This can take quite a bit of work, so be patient.

Mop up any remaining adhesive remover and give your MacBook Pro a few minutes to air dry.

The replacement battery included in your iFixit kit comes with adhesive pre-installed. Test the battery's fit and alignment carefully before peeling off the film covering the adhesive, and then press each cell firmly into place. If any additional films/liners are present that weren't on your original battery, remove them now.

Calibrate your battery before using it: allow it to drain overnight, then charge it to 100% and drain it again until your MacBook Pro shuts down automatically. Charge it again and use it normally.

If you notice any unusual behavior or problems after installing your new battery, you may need to reset your MacBook Pro's SMC.