How do you use and add FastFlags in Roblox?
To use FastFlags in Roblox, add them through a FastFlags editor instead of editing ClientAppSettings.json by hand. An editor lets you pick a preset or paste a JSON block, checks it is valid, and writes the file for you before Roblox launches. That avoids the two things that usually go wrong with manual edits: a typo that stops the client from starting, and a config path that moves with every version update.
BetterStrap, the Better Roblox bootstrapper, has that editor built in, with import and presets so you are not writing flags from scratch. See the Roblox bootstrapper page for how it launches the game with your flags applied.
Add flags with an editor, not the JSON file
Manually, FastFlags live in a ClientAppSettings.json file buried in the Roblox version folder. The folder name changes on every update, so a path that worked last week is gone this week, and a single misplaced comma stops Roblox from opening at all. An editor sidesteps both problems: it finds the right location itself and writes valid JSON every time, so your flags survive updates.
Import shared presets
Most people do not invent flags, they share them. Performance and graphics presets get passed around as JSON blocks, and importing one is faster than typing values in. In BetterStrap you paste the JSON into the editor and it validates before applying, so a bad paste is caught up front rather than breaking your next launch. Start from a trusted preset, then adjust to taste.
Reset when you want defaults back
The reason manual edits scare people is that undoing them means finding the file again. With an editor your flags sit in one panel, so removing a single flag or clearing everything back to Roblox defaults is a click. If a preset does not behave the way you hoped, you reset and you are back to a clean client with nothing left over.
