What you are looking for is the "BULB" mode. Set the camera to manual mode and cycle the shutter dial all the way to the left. After 30" the last option should be "BULB".
Once set on that, you press and hold the shutter button for as long as you wish.
Ideally I would recommend using a cable (RM-S1AM or RM-L1AM) or a cheaper option to get you started would be to stick the lens cap on and then tape the shutter button down. Once done, take the lens cap off and start the capture. Do the same at the end by putting the lens cap on then removing the tape.
After the photo is taken, busy will show on your screen for a while since it process the image. Be warned that it can take as long as the initial exposure itself so a 2 minute photo would take 2 minutes potentially to process so maybe try a shorter one to start with.
Hope that helps.
