Create a new project by clicking on Add Project. Select Create your own project.
You can now rename the project by clicking on the 3 vertical dots next to the newly created project and selecting rename,
Give your project a name and click on Rename Project.
Ensure the project is selected under My Projects. Now you can change the base data to aerial imagery.
Zoom to the area you need by by either using the zoom feature in the map or by holding down the shift key, left clicking on the map and dragging your cursor.
Click on Add Feature and search for Trees. You can now use the aerial imagery as a guide to place your trees by single clicking on the map. When you have finished you can switch back to Ordnance Survey data and you'll now be able to see the trees in your project.
Note: If you are completing a tree survey project, for example, you can add geo-referenced photos to a project. Please see this guide.