Ensure you have your mnemonic pass phrase for your wallet. This can be the 12 or 24 letter phrase you (hopefully) wrote down when you created you wallet for the first time. (If you've lost your passphrase, sorry...)
Using this offline tool, enter your mnemonic phrase. FOLLOW ALL SECURITY GUIDELINES EXPRESSED IN THE REPO ITSELF!! MAKE SURE YOU HAVE YOUR INTERNET CONNECTION DISABLED!!
Enter your BIP39 Mnemonic. Ensure you select BTC - Bitcoin in the Coin section.