Keep a way to enter and leave the layer
Choose an accessible layer key before placing the secondary actions. A held layer is active while its key is held; a toggle stays active until changed again. Keep an obvious path back to the base arrangement.
Leave commonly combined actions reachable together. For example, an arrow action should remain convenient while you hold Shift to select text.
Decide which keys change
A transparent position falls through to an active lower layer. A disabled position sends no action. Record which behavior you want for each unchanged position rather than assuming a blank legend defines it.
Use the planner’s base, Fn and extra label tabs to document the intended actions. Apply the finished plan through the exact board’s supported configurator, then use the event inspector to test those actions.
Characters still depend on the input source
Firmware keycodes and operating-system characters are separate. A board can send the same physical keycode while different input sources produce different symbols. Match the operating-system layout when you plan punctuation or language-specific characters.