First of all, you need to go to the exchange and login into your account.
How you should login into your account you can read here, if you have wallet model or here,if you have account model.

After that you you will get on the Dashboard page (Read more about Dashboard ). You can select the pair you are interested in to trade directly from this page or you can choose trading pair via "Exchange" tab.

For example, to open ETH/BTS pair using Dashboard you need to select ETH and then you need to click on BTS icon. 

You can also go to the "Exchange" tab and select the desired pair in the "My Markets" window

After that you will be redirected to the ETH/BTS market page

Let's take a closer look at the "Exchange" window.

A) Selected Market

B) Price chart.
C) Order window. Here you can enter orders to buy or sell coins.
D) Current market trades

E) Order book. Here you can see all orders of limit sellers and buyers.

F) Your limit orders.

G) Market selection window. Here you can select the markets you are interested in and add them to your "Favorites"

Let's try to buy ETH for BTS.

First, select the price at which you want to buy. You can fill in the "Price" field yourself or choose the price by clicking on the "sell limit order" placed in orderbook.

After that you have to fill the "Amount" field. The amount of ETH you want to buy.

Now click "Buy ETH". After that you will see a confirmation window. 

Please make sure that the entered data is correct and click " Confirm"

Now you can see in your account history that your order has been placed and executed