Create a new branch on your fork. You need it to be synced with the other fork so there are a few extra tricky steps. On your new branch, you need to delete the latest commits that aren't merged yet so that it matches the original repo. Then add a remote for the other fork and pull. Now you can build against the other fork and submit a PR to it.

git checkout -b even-cooler-stuff
# Remove the last 8 commits. Change this number as needed. Increasing it "too high" is just fine 
git reset HEAD~8 --hard
git remote add even-cooler-stuff https://github.com/more-of-url/even-cooler-stuff
git pull even-cooler-stuff

You should now have a branch that matches the other fork. Make your changes, commit, and push normally. When you build the PR, you want to merge into the other fork.

Disclaimer: I wrote this on my phone and from memory. There are probably typos and possibly other mistakes. Good luck!