For withdraw your steemit doller you must need bitcoin address.
you can get this address from Here
After collecting your bitcoin address ,
go to your steemit wallet.

then click on the steem doller.
select the sell option.

you will get a popup link and
there you will found option for sending steem doller and getting bitcoin.

Input your steemit doller which amount you want to withdraw.
then input your address in the "bitcoin address".

If your addressed is valid you will a massage on this plateform.
From there copy the username and the memo.

Now go to your steemit wallet and click on steem doller and select
"Transfer" option

Then you will go to a popup
there you can see they wanting a username
where you want to transfer
Usually the Username is "blocktrades".
input this username.

then input your amounts how much you want withdraw.
then on the "memo" field " past the memo
which you have copied from "blocktrades.us".
After inputing memo click on the "Submit" option.
After 5-10minutes you will get your payments in your bitcoin wallet.

Thanks for reading this post.
if you have any problem to withdrew your steem doller pls comments here.