To insert a tell block into your script:

Select the lines to be enclosed in a tell block. You can also choose to insert an empty tell block into the script text by placing the insertion point on a blank line.

Control-click in the selected area.

Choose the desired tell block option from the Tell Blocks submenu:
Tell “Finder": Encases the selected script text within a tell block directed at the Finder application.
Tell “System Events": Encases the selected script text within a tell block directed at the System Events application.
Tell Application: Encases the selected script text within an untargeted tell block.
Using Terms Clause: Encases the selected script text within a using terms clause.