When first being taught to visualise, I was taught via the first method, using the black disc. However after many years of doing research, I found a lot of people felt comfortable visualising a white light instead of using blackness, so I have created a second method which is specifically a white light variation for the third eye, and the original black disc.
First Method
Find a place where you will not be disturbed, away from any distractions
Get into a comfortable seated position
Close your eyes and slowly steady your breathing
Focus your energy on your third eye chakra
Slowly visualise a black disk expanding from your third eye
Visualise the disk expanding from your third eye
Expand it out as far as you are able
Continue this meditation for 10 minutes before opening your eyes to end it
Second Method
Find a place where you will not be disturbed, away from any distractions
Get into a comfortable position, this may be sitting down, laying etc
Close your eyes and slowly steady your breathing
Focus your energy on your third eye chakra
Slowly visualise a white light radiating from your third eye
Visualise the light expanding out from your head. If the brightness of the light begins to lessen, retract the expansion of the light until it becomes bright again
Continue this meditation for 10 minutes before opening your eyes to end it