i think most exchanges work the same. so coinbase is probably the same as bittrex. 2 years ago when i was trying to make an acc on exchanges, because i am from serbia and i was looking for most known exchanges i managed to open an acc only on bittrex.
So here is an advice from someone that has very little experience but did it in the last few weeks :D
Go to your Holdings page (i will write names from bittrex, not sure are them the same on coinbase) and look for steem. there will be two options Deposit and Withdraw. Click on Deposit, new window will open with memo and account. Then go to steempeak and click Send. In the new window copy the memo that you got from bittrex (coinbase), copy the account that you got from bittrex (coinbase) write the number of steem you want to transfer, click send and wait for few minutes.
I would do this first with 1 Steem, just so you could test it and see did you did everything right. If you have your 1 steem on the exchange, then do it all again for more steem.
Currently you can transfer that 800 steem and steem that is in SteemPower you need to powerdown first :D
If you don't want to change that steem to BTC, Hive or something else, maybe you don't need to move it. it is all on you :D
Important: memo is important when transferring to exchanges because they use one account for all transfers and memo is the info that connects it to your account on exchange.
if you need any help, write.
edit: now thinking, not sure that coinbase has steem listed.
Cheers for the run down man! I’ve just signed up for Bittrex.
Yeah Coinbase doesn’t support Steem bit it is listed as an asset that you can watch
hope it works :D if you need help, write