You have a few options
Bramblin is a new Pokemon introduced in the era of Pokemon Scarlet & Violet, and thankfully, it’s not too fussy in terms of evolutions. Not only can you easily evolve a Bramblin manually: but you can also just straight up find its evolved form of Brambleghast.
Where to find Bramblin in Pokemon Scarlet & Violet
This appropriately classified ghost/grass Pokemon can be found in the direct western and eastern regions of the Paldea map. Note that these are very common Pokemon, and hang out in deserts/arid areas.
Expect to run across a ton of Bramblins throughout your travels.
How to evolve Bramblin into a Brambleghast
If you want to evolve Bramblin, you’ll need to make it your lead Pokemon, and walk around with it for at least 1,000 steps. Then, level it up (you can use an XP or rare candy). Just make sure to put it in your top spot, then press the R button to have it come out and walk with you.
It doesn’t matter where you walk: you can meander around a town, and it’ll pop. Do a little extra legwork (at least 10 minutes) to make sure you hit this target. You may recall this evolution method from the Pawmo line.
Where to find Brambleghast
Alternatively, you can just find a Brambleghast out in the wild, as it has a natural habitat on Paldea. This will save you the time and effort if you have access to the above areas: which are typically seen in the late game (northwest and northeast).
These are rarer spawns compared to Bramblins, so just keep that in mind. By the time you reach these zones, you may as well have just evolved Bramblin yourself.
Related: How to get Iron Hands in Pokemon Scarlet & Violet on GameSkinny