In Bittrex go to Wallets
Search for ETH
Press "+"
Press generate New Address (This is the destination address at Bittrex where you'll send the Ethereum)
Copy the address (this is the destination address)
Go to https://www.myetherwallet.com/
Click on Send Ether & Tokens
Select Private Key
Insert your Private Key in the red box
Press Unlock
Complete Destination address
Complete Amount to Send
Click Generate Tranzaction
Confirm
Check your Bittrex Wallet