Once you have found the offer that matches your requirements, you can add it to your basket by clicking on the basket icon.
You can add as many offers as you like to the basket.
Once you have selected the offers that interest you, you can click on "order".
In the basket, you will find the offers you have selected and you must allocate them to a campaign by clicking on "attribute" on the right of the offer line: 
Once each line has been allocated to a campaign, you can proceed to payment.
It is also on this page that you can add options, for example the option to delegate the writing of an article (for an advertorial offer), or add extra words (800 words desired when the offer only includes 500, for example).