Well it is simple.
First you go to
https://www.poloniex.com and create a account.
Then you go to Balances -> Deposits and Withdrawals and find XMG
Now you click Deposit on the XMG and you will get an address, you then send the coins to that address from your wallet.
Once that is done you wait for them to confirm and go to
https://www.poloniex.com/exchange#btc_xmgThere on the Right side is SELL where you set the price you wish to get and the amount you want to sell.
You will also see the Bids for XMG of other traders and the offers of others in the tradebook under the trading area, this will show you how much people are willing to pay for XMG.