I actually don't know of a tutorial, but it's very straight forward.

Go to: TribalDex

Choose POOLS
Choose Add Liquidity
Select the DEC Pool Pair you wish to add liquidity to, i.e. DEC:SPS, DEC:BEE, DEC:SWAP.HIVE, etc...
Select the amount of each you want to add, and click the big execute (Add Liquidity) button below. BOOM!! you're officially a lender!! Welcome to the world of finance!