Practice.. but night exposures are very forgiving. You just need to remember that the difference between 1 minute and 4 minutes is only 2 stops.. or the same as the difference between 1/250 and 1/1000th of a second.
You can set your ISO to 800 or 1600 or higher and take your meter reading through the camera, maybe even fire off a test shot. Then bring your ISO back down to 100 and add the extra stops back onto the shutter speed.
So.. keeping the maths easy.. at ISO1600 you metered the scene at 5 seconds and a quick test shot looked about right on the lcd screen (for what you had in mind).. you put the ISO back down to 100 and change the shutter speed to 80 seconds (doubling the shutter speed for every halving of ISO on the way from 1600 to 100). You can time the 80 seconds with your watch and use a regular remote shutter, or you can use a remote shutter with a built-in count-down timer for doing bulb exposures. Being bang-on 80 seconds isn't going to be critical.. 5 seconds either way really won't make much difference.