Yup. If she is sitting on eggs pretty much 24x7, puffying whenever anyone comes near, and cranky, she's brooding.
I would wait about 5 days to make sure a new hen is serious, then get fertile eggs.
You can set up a partition or move the hen to a new area with some dud eggs. That is done best at night during the trial period before you buy fertile eggs.
I do recommend isolating hens in a new situation especially if the flock is not conditioned for coop brooding.
If she is serious, and you've got her moved successfully, or blocked away from the other hen, then set fertile eggs at the same time marking the set date on the eggs.
At that point, it is letting momma do her thing in a nice, quiet, comfy nest, free from distraction. Momma will need access to food and water and a dusting area. She will take advantage of that for about 15 to 20 minutes once a day, almost always when you are not watching, so don't panic that she doesn't appear to be eating or drinking.
Also be aware that brooding hens do lose body weight and do have paler combs due to the process and hormones.
In 18 days, be sure things are "locked down" for no interruptions, meaning your hen likely will not get up and should not be disturbed. In coop brooding, I block the nesting hen for those crucial days and the first 3 days of chicks.
Really, really resist checking and looking too much or you can disturb the process. Hens when left to do their thing generally do really well raising babes.
After that, the babes will be up with momma about day 3. Be sure they have a safe place to roam so that they can't get stranded away from momma. The little dears can always find the hole in the fence out but never seem to find it back in.
Food for momma and chicks is simply the chick start or all flock. Avoid layer feed with the chicks until the hens are ready to lay.
And don't feel you need to interfere with momma as she clucks and calls chicks. You don't indicate your geographic location, but I've successfully broody hatched in all types of weather including the most frigid and the babes do fine with momma as long as they don't get stranded and are out of direct weather. No heat lights necessary. It is amazing....sort of like they were designed to do this
Have fun with your broody. Let us know how things are going. There is a great broody thread on BYC with LOTS of info (don't feel you need to read the 1,000 some pages).
https://www.backyardchickens.com/threads/broody-hen-thread.496101/
LofMc