If you've thought about powering down your account so you can spend your funds but aren't sure about the mechanics, here's a little guide of what I've found works for me. There are several services available to act as exchanges and online wallets; the ones I mention are the ones I use and have experience with. I cannot vouch for other services.

1 - Get Your Steem

You've written, photographed, archived, found something nifty or created something awesome. Posted, commented, and resteemed. You've earned some Steem Power and Steem Dollars and you're ready to make the most of them, financially speaking.

The important thing to note is that you will not be receiving a lump sum: you will get 13 weekly payments. So once you hit Power Down, you've gotta wait. My understanding is that is in place so when people cash out, it doesn't crash the currency's worth. As a side note (from my own experience) : the amount of Steem will remain the same ( 1/13th of the amount being powered down) but the worth of the Steem will vary from payout to payout. For example, my first payout was about $40. Second, $60. Third, $160. It fluctuates like the stock market. If it goes up, expect it to go down-- just hold onto the Steem until it picks up again and you're ready to spend.

If you want to convert Steem Dollars into Steem, click on MARKET and buy Steem.

Once you have Steem you want to spend (or whatever) you need to get yourself an account at a Bitcoin Exchange. Once you have an account, you will be issued a special code. This is akin to your mailbox. Copy, paste into a text file for safekeeping.
I use Poloniex. Haven't had any issue with them.

2 - Transfer Steem to Exchange Service

You're sending it to Poloniex for them to deposit into your account. That special code goes in the memo section of the transfer window, so Poloniex knows to whom it belongs.

It will take a couple minutes for it to make it from Steemit to Poloniex.

Once its there, head on to the exchange.

3 - Exchange Steem for Bitcoin

4 - Transfer to Bitcoin Wallet

I like Coinbase; but there are many out there.

Like Poloniex, you'll get your own special bitcoin address (and the ability to make many more) where you will receive funds.

5 - Spend! (or save... your call)

I like Coinbase because they work with the Shift card; it's basically a debit card for your Coinbase account. So, once things clear (it's about an hour, start to finish) in the Coinbase account, it's available for spending.

There are options available, it's my suggestion to read up on whatever services you choose, that way you're forearmed with knowledge.

Gotta say, it's pretty nifty that my writing treats my family and I to dinner out every now and again.