I managed to replace a swollen battery ($33.25 on amazon) that popped the screen, but this is not for the faint of heart. Special emphasis needs to be placed on not twisting the digitizer connectors as they are quite fragile ($119.99 on ebay for a new digitizer). It's also easy to damage the force-touch sensor ($24.95 on iFixit). You also need adhesive gaskets ($12.98 for 5 on Amazon)
iFixit lists all the parts required but were out of stock on all but one of the items I needed at the time.
Force Touch Sensor Gasket:
Apple Watch (38 mm Original and Series 1) Force Touch Sensor Adhesive Gasket
I can't stress enough how important it is to not flex the digitizer ribbon cable. A cracked ribbon cable will necessitate replacing the entire lcd.