Once you've got your SLP tokens from being a "Liquidity Provider" in the SushiSwap Pools (read the guide here) then you are ready to also stake your tokens in the SushiSwap Farm (Menu) for additional Sushi Rewards every block.
Navigate to https://sushiswapclassic.org/farms and unlock your wallet.
Select the pool you want to stake into:
From here you'll need to "Approve" the SLP token for the first time:
Once the token is approved you'll be able to stake your tokens by using the "+" button.
This will then display a pop-up asking you how many SLP tokens you wish to stake. Enter the amount and hit confirm
After this you'll see a transaction as you wait for your token to be staked. Once staked it will appear in the farm and you can unstake it at anytime. Important Note: When you deposit your SLP for farming, the exchange.sushiswapclassic.org/#/pool page no longer sees it. That page only scans your wallet to see if you are holding SLP tokens. So that page will say "No Liquidity" - however you do still have your tokens and you are still earning the 0.25% trade fee, it is just currently deposited in the farming contract and so not detected.
After this, your "Sushi Earned" balance will update every block. You'll be able to click "Harvest" at any time to bring out your Sushi.