Navigate to Settings > Configure system > Toggle Features.
Click Invoicing and Expenses on the left-hand side and add a tick to the checkbox beside P11D Expenses and Benefits.
Navigate to Purchases > Expenses.
Click on the Add receipt button.
There will be a new tickbox to include the expense into the P11D form, as shown below.
In order to see the items in the P11D form, toggle it on then navigate to Reports >All Reports and click on the P11D items link under the Tax header.
On this screen, you will see the employee expenses which have the P11D ticked. Items which are included in the P11D are on the upper table and excluded items are on the lower table. Ticking the boxes on the upper table and clicking Update button will remove the items, while doing the same at the bottom will add the items.