Here are my recommending steps for starters. Feel free to compose your own one. For this one I shall brief on FCEUX the NES emulator as an example and generalise on graphic editing.
Requirements:
Download the FCEUX emulator from sites that can be located via a search engine and the roms of the games you want to map.
1. When you've loaded your game, you can start playing through to begin. You should also disable the sprite layer in preparation of background screenshot capturing (you can always turn it on if you have to go further in the level)
2. From the start of a level, press F12 to capture a screenshot which will be saved in the 'snaps' folder. It is always a good idea to pause the game while capturing screenshot in case an early animation, character death or such like interrupts you from getting the desired screenshot.
3. Once you've got the screenshots, you can open the first and second screenshots with your graphic editing program (you can open more if you want, but two saves memory). You may have to delete bits and pieces from the screenshots like menus, icons and such like.
4. Take a chunk of the second screenshot and paste it like a jigsaw on the place it fits on the first screenshot. While you're putting those screenshots together make sure you align them properly.
5. After several screenshot cutting and pasting and hopefully completing the layout of the level, take note of where sprites are (precisely or approximately) and capture screenshots of the sprites before cutting them out and pasting them in the right place on the map. You may need to whiten the colours around the sprites if you want to copy and paste them multiple times to save time.
6. When you've got the map finished, ensure that the edges of the map are at the edge of the graphic editing program screen, or you can leave a tiny space between the edges for a border.
