How? I just get "Create branch: <branchname> from '<currentbranch>'".
Oh. I can't really test this because I'm a collaborator, but try opening Jens's project in Github Desktop, then make a branch.
Oh. I'm sure it can be done in command-line git, but I have no idea how. Anyone?
That's another problem: I don't have Git either.
However, I'm willing to look at my commit(s) about bignums (and my commits on a PR to djdolphin's fork, making scripts in help screens able to be grabbed from the help screen) and copy the file by hand, then delete my fork, then refork.
Edit: I just reforked it, and restored my PRs. (bignums, dragging block out of help screens) I also made a new branch for help screen fixes. I'm also not going to modify master beyond just merging Jens's commits to it.
Can I suggest this method
Make sure you have a seperate local copy of any changes not yet pushed back up to jmoenig/master for a PR.
Delete your fork - re-fork
Create a new branch - give it a meaningful name and make sure your switched to it.
Make your changes in Github - create a PR when ready to do so.
Do not make any further changes in that branch until your PR is accepted.
When it it accepted - delete that branch and update your fork from jmoenig/master
You can probably work on another branch while waiting for Jens to accept your 1st PR but I've never done that myself
No need to suggest it; that was my plan--separate branches for each PR. Except
would have one exception: merge branch 'jmoenig:master' into <PR branch name>.
You don't have to do that. Git will remember the base from which you modified stuff, figure out what changes you made, and make the same changes in the current version. Git is really smart except for when it thinks it's smarter than you are.
I don't want to risk it being smarter than me. So I
Your on your own with that approach 
I just keep things VERY simple when using git(hub) as I've found that I can end up in a right pickle if I try and be clever
In seriousness, there are good reasons (explained to me by a git expert that I didn't really understand but I trust the expert) for just keeping your branch separate and deleting it after PR accepted and only then re-syncing your fork with the main repository
But up to you
see
They are just (I presume) editing a local, non-git copy and then replacing things in their github repository
I'm confused; whom do you mean by "they"?
No, I'm using the online editor.
Me.
warped_wart_wars
...............
Yeah, I'd delete the PR branch after it's merged, but I'd merge jmoenig/master into my branch before it's merged if there are changes to jmoenig/master.
But there's no need to do that, because git (even if you're using it web-based rather than on your desktop) remembers where the master was up to when you made your branch from it, and it'll take care of all that. I think you're actually making its job harder by adding edits to your branch that are also added in the master!
That's what I'm STRONGLY suggesting not to do
One day - it'll bite you in the backside and you'll go - oh - Brian and Simon were right
More likely, I'd think "bh (or Brian) and cymplecy were right".