When you have identified a test you wish to order, click on the "Order test" box. This adds the selected investigation to the list of orders, and the test is highlighted in yellow. The indicative cost of the test is also added to the running tally at the right of the page.
When you have finished selecting tests you wish to order in this encounter, you can proceed to the next tab. Note that requests for the tests you have selected to order will only be submitted when you click the "Submit orders" button. Until you do so, you can remove selected tests or return to the test ordering screen to add investigations to this encounter. You are allowed a maximum of 5 test/review encounters.