A hardware colour-calibrator such as a Colormunki, or Spyder. Equally, if not more important is the screen angle. I keep a zone-system chart on the desktop of my laptop and I always make sure that my screen is angled so I can see the ten distinct graduations of that before I start doing any processing on it.
But the problem with a laptop is that most of them use a 'TN' type display, which looks different depending on the viewing angle....and because laptops have a folding design, and because they are portable, the viewing angle isn't consistent. So even if the display is calibrated, you may not be seeing it consistently, which would cause inconsistencies in your editing.
Also, because it's portable, you are much more likely to use it in various locations with different types and amounts of ambient lighting.
So the best solution, if you must edit with a laptop, is to have a separate monitor that you use for critical tone/colour editing.
Yes. You need two things.
1. Calibration device. Personal choice is the Color Munki
2. Get a standalone IPS monitor. Laptop screens are not good choices for editing.
Or have a friend who owns one calibrate your monitor and printer for you. you do need to do this more than once though as monitors drift, so owning your own is best. Maybe try finding one used.
Yep. Displays need to re-calibrated regularly - like once a month.
If the ambient light falling on a display has changed, the display has to be re-calibrated, which is one reason why moving a laptop around is problematic if you want accurate colors.