1. Go to your PiHole Admin at pihole.example.com/admin
2. Go to Group Management >> Addlist
3. Add the list you want there (e.g. https://dbl.oisd.nl)
4. Click on the link "online" above, or go to Tools >> Update Gravity, or pihole.example.com/admin/gravity.php
5. Update the database

Now you can check on the start page, there should be about 1Mio blocked domains.
I added https://dbl.oisd.nl, see https://oisd.nl/how2use