Two ways, depending on where the config comes from:
Community configs (recommended) — nothing to install, just load them in game:
.config list
.config load <name>
A config file someone gave you — it has to be a LiquidBounce .json file:
.localconfig browse — opens the folder LiquidBounce reads local configs from.
Drop the .json in there directly (no subfolder, and watch out for Windows hiding the real extension, so you don't end up with name.json.json).
.localconfig list to check it shows up, then .localconfig load <name>.
If loading errors out, the file usually isn't a valid nextgen config (a .txt, or one made for another client/older build) — renaming it won't help. Also make sure your client is on the current release; old builds fail to load configs that otherwise work fine.
All of the config commands are listed here: https://liquidbounce.net/docs/commands/client
Automated support reply. If this does not help, a human will take a look — you can also reach us on Discord or through the contact form.