Find a village with a lectern and a librarian villager
Break the lectern if the librarian does not offer Mending
Place the lectern again to reset the librarian’s trades
Repeat until the librarian offers a Mending book
Trade with the librarian to lock in the Mending trade
Keep the librarian safe so it does not die or lose access to its workstation
If needed, cure a zombie villager and turn it into a librarian for cheaper trades