Controls:

F: toggle flying on/off
W / A / S / D: move relative to where the camera is looking
Space: fly up
Left Ctrl: fly down

Follow these steps:

Open Roblox Studio and open your game (or create a new Baseplate).
Turn on the Explorer panel: if you don’t see it, click the View tab at the top, then click Explorer. Also turn on Properties, it comes in handy.
In Explorer, find StarterPlayer and click the small arrow next to it to expand it. Inside you’ll see StarterPlayerScripts.
Right-click StarterPlayerScripts, choose Insert Object, then LocalScript. (Or hover over StarterPlayerScripts and click the small +.)
The new LocalScript opens in the editor with the code print("Hello world!"). Delete all of that code (Ctrl+A, then Delete).
Open the file FlyScript.lua. You can open it in Notepad, copy everything (Ctrl+A, Ctrl+C), and paste it into Roblox Studio (Ctrl+V).
Start the game with the Play button (the green arrow at the top, in the Home tab).
In the game, press F and you’ll start flying. Controls: W/A/S/D to move, Space to go up, Left Ctrl to go down. Press F again to turn flying off.

Important: the script must be a LocalScript (not a regular Script) and it must be in StarterPlayerScripts. Otherwise it won’t work.