Use American English spelling, but mostly British English grammar.
Try to write in a simple, precise and concise manner. Non-native English speakers are likely to be reading the documentation, so it must be easy to understand for as many people as possible.
Open a menu
Press a button
Select a menuitem
Application menu: Open the <app> menu from the top bar and select <menuitem>. (Note: Application menus have been deprecated in GNOME 3.31.)
- system status area: the right end of the top bar which displays status icons
- system menu: open the system menu from the right end of the top bar
The top window bar (which includes the title bar plus might include some buttons) is called header bar.
Generic window menu: Press the menu button in the top-right corner of the window and select <gui style="menuitem">entryname</gui>.
Switches: Switch the <blah> switch to on/off.