Once the trade request has been accepted the trade screen will show up.
In order to add an item to the trade window you'll need to select the inventory it belongs to. This can be done by selecting the drop down menu at the top left.
Once you've made a selection the inventory you selected will now be displayed underneath the drop down menu. You can navigate through your inventory by using the arrow buttons and you can search for a specific item by using the search bar and filters underneath it.
To add an item to the trade drag and drop the item thumbnail into the top half of the offers box. The chat window at the bottom left of the trade screen also serves as a log file which documents what is added and removed from the trade window.
Once the other person has added their item to the trade window click on the blue
ready bar underneath your offer. The bar will then turn green
indicating to the other person that you've finalized your offer.
If you'd like to make an adjustment to the trade you can click on it again to remove the ready status. The ready bar will automatically revert back to blue if the other party makes a change to their offer.
Once both people have marked themselves as ready the Make Trade
button will light up, to commit to the trade click on it.
Once both people have clicked the Make Trade
button the items will exchange hands.
You'll now be able to view your newly acquired item within your Steam Inventory.