To use a Ledger hardware device to manage your Keplr wallet, first ensure you have the Cosmos app open and unlocked on your Ledger device. Select Import Ledger in the Keplr UI and follow the on-screen prompts to connect your Ledger to Keplr.
Ensure that you store your mnemonic seed phrase somewhere safe where it cannot be lost or compromised. Your mnemonic seed phrase is the master key to your wallet; loss or compromise of your mnemonic seed phrase may result in permanent loss of your ATOM.