On the DIESEL POOLS PAGE of TribalDEX.com, click on the first tab in the list, entitled "Swap."
Select the token to swap From, and the token to swap To (pictured below: SWAP.HIVE -> DEC)
Enter the quantity of the token to be swapped in the box to the left. The To field will then be filled in with the amount of DEC you will receive, with current prices and liquidity for both tokens displayed.
Set the Max Slippage by choosing a percentage - Slippage is the difference between the expected price of a trade and the executed price of that trade. This is typically only important with extremely volatile and high-volume tokens.
Double-check the swap's details, then click the SWAP button and sign the transaction with your Hive Active key using Keychain.
If the swap is successful, you will receive a green notification and you will then see the new token added to the appropriate balance in your wallet.
Note: The wallet on TribalDEX is an alternate interface to the same wallet you'll find on HIVE-Engine (Your Hive Wallet). For instructions on transferring these DEC into Splinterlands, visit THIS PAGE.