To donate troops in Clash of Clans, you first need to join a clan. Being part of a clan allows you to both donate and request troops. Next, upgrade your Clan Castle. A higher-level Clan Castle, especially level 8, lets you donate a wider variety of troops.
Once you’re in a clan and your Clan Castle is upgraded, you can request troops. Use the “Request” button at your Clan Castle or the Barbarian button at the bottom of the Clan Chat. When requesting, you can specify the types of troops, spells, or Siege Machines you want. If you don’t specify, your clanmates can donate any type of troop.
To donate troops, tap on the “Donate” button next to a clanmate’s request in the Clan Chat. Select the troops you want to send from your army. Donated troops are great for attacking and defending in Clan Wars, multiplayer battles, and Clan War Leagues.
Do note that donated troops can only be used once. They won’t return to the Clan Castle if they survive an attack. For Clan Wars and Clan War Leagues, you must request troops separately for defending your war base.