First, send HIVE to TribalDex or Hive Engine. Your HIVE is converted into SWAP.HIVE:
tribaldex.com -> login with Hive Keychain - > Wallet -> Deposit -> Select a Token -> HIVE
Then, send DEC from Splinterlands to TribalDex or Hive Engine:
https://splinterlands.com/?p=sps_management -> Transfer -> SELECT TRANSFER TYPE -> HIVE ENGINE -> Enter Amount -> TRANSFER OUT
Next, add liquidity to the pool:
tribaldex.com - > Pools -> Add Liquidity -> SWAP.HIVE:DEC -> Input the amount you want to add. (Must be about half Hive and half DEC.)
Alternatively, you could just send DEC to Hive Engine / TribalDex, and then trade half or the DEC for SWAP.HIVE and then add to the liquidity pool.
Good luck!