Go to wefwef.app in your browser. Now open the options menu of your browser (the three dots, if on Android, the square with the arrow pointing up, if on iOS) and find the option to put a bookmark on your home screen (in iOS it's called "add to home screen")

This will create a sandboxed version of wefwef.app, tap it and it should load like a normal app. (this is what Apple envisioned as apps way back when, before they realized that they could make money selling apps)

When the page has loaded, in the bottom middle, tap on "lemmy.world", and log in to your Lemmy instance.

Enjoy.

One thing to be aware of. If you have selected more than one language in your Lemmy profile, wefwef fails to post comments because you need to select which language you typed in, and you can't select that (yet?) in wefwef.