In Bittrex go to Wallets
Search for SIA
Press "+"
Press generate New Address (This is the destination address at Bittrex where you'll send the SIACOIN)
Copy the address
Open SIA-UI aplication
Go to Wallet
Enter your passphrase
Press Transfer
Insert the destination address and transfer amount (consider some SIACOIN will pay the transaction)
Wait..
Check your transaction here: https://explore.sia.tech/ or in your Bittrex account
Wait...