actual fullscreen on melonds tutorial:
1. open up your melonds
2. go to config on the top left corner
3. while on config, click on input and hotkeys
4. while on input and hotkeys, go to general hotkeys
5. find toggle fullscreen and click on the typebox to press any keys on your keyboard
6. click ok once after you press your key on your typebox for your toggle fullscreen control
7. go to open rom or open recent to click on any games you want
8. now press the key depending what you chose for the control of the toggle fullscreen and enjoy your fullscreen ds gameplay!
hope this helps!
(i would recommend the f11 button for the toggle fullscreen control because like i mentioned on this video, f11 was very useful especially when you used f11 for fullscreen on youtube videos)
melonds download: https://melonds.kuribo64.net/
enjoy the video!