You generally want the coins to be in a single use wallet you can extract the private key from after you return the funds to your main wallet, after fork. Jaxx may provide other support but in general I don't use a private key from a main wallet/account for safety. Using the key from an empty desktop wallet that you can reset later, seems to be ideal.
1. Transfer coin(s) to Desktop Wallet prior to Fork, save/backup
2. After fork Transfer coin(s) back to Main/Safe Wallet/Account
3. Download & Sync New Fork Coin Wallet
4. Extract Private ID from Empty ETC Desktop Wallet (funds should have been returned to main wallet AFTER fork)
5. Enter Private ID into New Coin Fork payment area
6. After entering Private ID, New wallet will ask to rescan. Say Yes
7. New coin wallet should resync/scan and your forked coins will appear
(Always make sure to make backups, save phrases and passwords in a safe place and Never use a Private ID from a wallet with funds, practice safe forking)
Hope that helps