The guides are missing the part where you put back the back cover

Typically you peel off the old adhesive and replace it with new stuff. Don’t use silicone, use something like Tesa black tape. Some parts suppliers will sell precut adhesive for most devices. Some suppliers will also sell back covers with new adhesive either already attached or you need to apply it. In some cases, if you cut through the adhesive clean enough you can reuse it. Just put the back cover back on, use some heat from a heat gun or hair dryer, and then either clamp it down for about 20 minutes or put some heavy books on it.