It’s simple, really.

• if you have Reddit app installed, uninstall it first
• download revanced manager and install the apk. https://revanced.app/
• find Reddit app’s apk in apkmirror.com and download it
• open revanced manager and select Patcher -> “select an application” -> storage and select the downloaded Reddit apk.
• make sure “disable ads” is in the list of selected patches
• run the patcher, wait until it completed
• don’t install the resulting apk right away yet. Instead, open the “…” menu on the top right of the screen and backup the patched apk
• close the revanced manager app and open your file manager. Locate the patched apk you just backed up in the download folder, then install it.

Note that I haven’t actually tested this with actual Reddit app.