Install with the CurseForge app — LeiCraftClient Docs

The CurseForge app only installs mods that are on CurseForge. LeiCraftClient is not on CurseForge, so you create a Fabric profile in the app and copy the LeiCraftClient jar into its mods folder yourself. It takes a minute longer than the other launchers, but it works the same.

What you need

The CurseForge app with Minecraft set up in it.
The LeiCraftClient jar from the download page.
Fabric API for 26.3 and Fabric Language Kotlin 1.14.2 or newer. You can add both through the app, or download them from Modrinth (Fabric API, Fabric Language Kotlin).

Install

Create a custom profile

In the Minecraft section of the app, click Create Custom Profile (in some versions of the app: + Create).
Enter a name for the profile.
Choose Fabric as the modloader. The app also offers Forge, NeoForge and Quilt; pick Fabric.
Choose game version 26.3.
Click Create.

Add Fabric API and Fabric Language Kotlin

Open the new profile and click Add More Content. Search for Fabric API and install it, then do the same for Fabric Language Kotlin. Make sure you get versions for 26.3.

If you downloaded them from Modrinth instead, copy them into the mods folder in the next step together with LeiCraftClient.

Download LeiCraftClient

Download the LeiCraftClient jar from the download page. Don't unzip it.

Open the profile folder

Open the profile's menu (⋯) and choose Open Folder. The exact position of this option can change between app versions; look for it in the profile's menu or settings.

The profile folder opens in your file manager. Inside it is a folder called mods. If it isn't there, create a folder with exactly that name.

Copy the jar into mods

Copy the LeiCraftClient jar into the mods folder. If you downloaded Fabric API and Fabric Language Kotlin by hand, copy them in too. In the end the folder must contain all three mods.

Start the game

Start the profile from the CurseForge app. Look for the LeiCraftClient button on the title screen, below the Realms button. If you see it, LeiCraftClient is installed.

The CurseForge app can't update LeiCraftClient for you, because the mod is not on CurseForge. When a new version comes out, the mod tells you on its options screen. Download the new jar from the download page and replace the old one in the mods folder.

Next: link your account

You can already see other players' capes and cosmetics. To wear your own, link your Minecraft account: Link your account.

Troubleshooting

The game stops with an error about missing or incompatible mods. The error names the missing mod. Most often it is Fabric Language Kotlin or Fabric API. Add it with Add More Content, or copy the jar into the mods folder.
There is no LeiCraftClient button. Check that you copied the jar into the mods folder of the right profile, and that the profile uses Fabric and 26.3.
Two LeiCraftClient jars in the folder. Keep only the newest one.

More help: Troubleshooting.