Buy the Dowel, Sand the Ends, Don't Cut It Yourself
Okay, dowels. This one's short because there's not much to it, but people mess it up anyway.
Go to the hardware store and buy a dowel. Don't cut one from a branch in your yard, don't ask your neighbor with the table saw to rip one down for you, and don't grab the cheapest bin option without looking at it first. Just buy the dowel.
Here's why I'm particular about this. A rough-cut dowel, one somebody sawed off by hand without cleaning up the end, snags your cord. Every single time you slide a knot up near the top, it catches on some little bur of wood and you have to stop and pick at it. It looks it too. You can tell, on a finished piece, which ones had a clean dowel and which ones didn't. So unless you've got a real saw and you know how to get a square, clean cut, just let the store do that part. That's what they're for.
What to actually buy
A 3/8 inch or half inch dowel is plenty for a wall hanging. Length depends on your project, but for a first one, 12 to 18 inches is a good range. Wood, not metal, not the plastic closet-rod stuff. Plain pine is fine. You don't need anything fancy.
While you're there, this is a fine thing to pick up on a regular hardware run. You don't need a special trip. Grab it same time as anything else you're getting.
Sand the ends anyway
Even a good clean dowel from the store, sand the ends. Both of them. A little sandpaper, 120 or 150 grit, just enough to knock the sharp edge off and smooth out the cut. Takes you thirty seconds.
I'll be honest, I didn't do this for a while. Figured if the dowel came pre-cut it was fine as is. Then I got a splinter loading cord onto one, and it wasn't even a bad cut, just a normal store dowel. Our air out here is dry enough, especially in the winter, that wood gets a little more brittle and split-prone than you'd expect. A rough edge that would've been no big deal somewhere humid will catch your finger here. So sand it. Cheap insurance.
Now, about the cord
This is where I want to bring up something that's got nothing to do with the dowel directly but everybody does it at the same stage, so I'll say it now.
Once you've got your dowel ready, the instinct is to sit down and cut all your cord at once. Measure out your strand length, whack, whack, whack, cut the whole pile, feels productive, feels like you're making progress before you've tied a single knot.
Don't do that.
I did exactly that on a project a while back. Measured what I thought was a good length, cut every single strand the same, probably thirty of them, all set to go. Started tying my first row and about halfway through I realized I was running short. Kept going anyway, thinking it'd work out. It did not work out. Every strand came up about eight inches short by the time I got to the bottom of the piece. Every one. Because I'd made one small error in my math and then repeated it thirty times before I found out.
Had to cut all new cord and start over. Wasted a chunk of an afternoon and a chunk of cord, and waste is the thing that bothers me most in this whole hobby.
So here's what I do now, and what I want you to do. Measure your length, cut one strand, fold it on the dowel and tie one full row of knots with it. See how it actually looks and actually behaves before you commit. Then, once you know that length works, cut the rest to match. It feels slower at the start. It saves you every time.
A word on the fringe end
Don't trim your cord ends short right now either, even after you've got your row tied. Leave plenty of extra length hanging below where you think the piece will end. You'll trim all of that later, at the very end, once the whole thing is knotted and hanging on the wall where you can actually see what it needs. Cutting early is how people panic-trim a piece too short and can't fix it.
Before next time
Get your dowel and sand both ends before our next session, that part you can knock out in five minutes at the kitchen table. Hold off on cutting any cord until we're together and you can test that first row.