See our DEXes list for the list of official DEXes.
Once you are in Serum DEX, make sure you are on mainnet-beta on the top right corner in order to trade real markets.
Click Connect wallet (Make sure you have already switched to your desirable account in Sollet.io) and finish the connection process by clicking on Connect
And you will see your wallet balance synced with your Sollet.io balance.
You can disconnect your wallet at any time by clicking on Disconnect on the top right corner of Serum DEX.