Instructions
DeSmuME
Launch DeSmuME.
Click "View" at the top of the screen.
Scroll down to "Window Size."
Click "4x" to view your emulator on a big screen.
NO$GBA
Double-click the "NO$GBA" icon on your desktop. A window appears asking you to insert a cartridge from a ROM stored on your computer.
Load an NDS ROM from your computer.
Click the "two squares" icon in the upper-right corner to view the emulator in full-screen mode.
DSemu
Launch DSemu. You will be prompted to load an NDS ROM from your hard drive.
Search for and open an NDS ROM.
Move your mouse to the lower-right corner of the screen, where you will see three diagonal lines. Move your mouse to the edge of those lines, and then press and hold the left mouse button to expand the NDS emulator until it fills the screen.