It feels like not a simple thing to explain.
As an example someone sent me a PR with 15 commits. github claims only 1 file has changed and it's a new file so no conflicts (which AFAIK is correct) but git complains that somewhere around commit 4 there's a conflict.
The person sending the PR is not git comfortable, otherwise they wouldn't have this PR with 14 old unrelated commits showing up.
I have no idea what state they are currently in as well. meaning, I know the state of the PR but I don't know the state of their local machine.
I can clone their branch and make the merge myself but their next PR will just be the same branch they continue to work on.
Explaining to them how to fix what they have and how to proceed going forward seems like it requires a very large reply. Several pages. (here's how to fix this PR, this how to fix the state of your main/master branch, here's how to make different branches, here's how to update after each PR, here's what to do if the conflicts are real, here's what to do if you haven't made any more changes to your main branch., here's what to do if you have already made changes to your main branch.
I have no suggestions for solutions except to try to point them to other articles/pages/tutorials on the web