Instructions
Download and install an emulator on which to play your ROMs. If you don't already have a Nintendo 64 emulator, there are several good programs to choose from: this article will refer to the most popular Nintendo 64 emulator, Project 64, which is available for download in the Resources section.
Start a game on the working ROM of your choice. Play the game until you reach a point where you wish to save your game.
Open Project 64's System menu and select "Save as." The shortcut for this command is "Ctrl-S."
Type a name for your save state or use the existing default. Make sure not to delete or overwrite the ".SRAM" at the end of the file name: this suffix is how Project 64 (and other emulators) will know that the file is a save file that can be loaded. To accept your save file name, click "OK." Continue playing or close the emulator.
To load the game where you left off, open Project 64's "System" menu and choose "Load." You can also press "Ctrl-L" on your keyboard. Select the game file to be loaded and double-click it. Your game will restart exactly where you left it.