It is common for disassembly guides to end with "To reassemble your device, follow these instructions in reverse order." However, it's usually easier to take something apart than put it back together.

To snap the bottom cover back into place, you must start by aligning it with the audio jacks. Otherwise it won't fit into place.

If you are doing an upgrade, it is a good idea to disconnect the battery. This requires a lot of awkward wiggling of the connector. Before putting the bottom cover into place, do not forget to plug the battery back in.

This doesn't look easy and there are opportunities to mess things up. Also, it might be hard to find an unused analog stick cover. I would try silicone joystick caps from eBay before attempting this guide.

Sometimes plastic or rubber items I handle become extremely sticky and can't be cleaned. This is a very annoying problem for me. I remember my stepdad yelling at me because I'd made swimming goggles sticky, and he also thought I'd covered bicycle handlebars with glue to make my hands stick better. Really I think it's just resins and plasticizers leeching out because of my sweat. I had the same problem with my PS4 controller's joysticks. No matter how carefully I wash my hands or clean the controller they're sticky. If I were to sell it for parts I wouldn't want someone to buy my sticky joystick covers.

I bought some silicone joystick caps from eBay and they were easy to install, I just stretched them over my existing joysticks. They've even got some little bumps that help with grip.

Be careful that the bottom of the phone is well heated, and add a drop or so of isopropyl alcohol. When I removed the back glass of my S6, some of the black coating on the glass stayed stuck to the adhesive, so to return my phone to its original appearance, I’ll have to get a new back glass.