I found this very difficult.
The digitizer had cracked and was unresponsive in an area of the screen, so I tried to replace just the digitizer as the LCD was in perfect working order.
The digitizer needed separating from the LCD and the glue is very strong and needed to be heated for quite a while until it was malleable.
I didn't manage to get the glue between them to be clear (I ended up with a rainbow effect) and I managed to damage the LCD controller when reassembling the digitizer/LCD assembly.
The connector between the digitizer and LCD is very fiddly to reconnect, especially when folding it back over to seat it in the main housing; and the seal around the front panel was unsatisfactory.
I gave up in favour of a professional repair!
I used a domestic hair dryer for about 2 minutes to heat the back of the phone which softened the glue enough for the rear panel to be removed. A layer of glue remained around the frame. The replacement part also had a gummed edge and stuck quite firmly without needing any pressure.
I reheated the rear to soften the new glue and then held the phone in two small clamps gently but firmly until the phone had cooled (overnight as it was convenient).
The phone is waterproof.
Excellent guide, thank you.
I installed a 4TB WD Green with two partitions. Worked a treat.
Original capacity was 500GB, 2nd generation TC (2009).