telescope - How do we convert other waves of the EMS to visible light?

It's just as simple as taking the flux at some wavelength (just a number) and using this number to represent a visible intensity.

If you only have one wavelength then you can only get a monochrome picture. However, if you have flux information at more than one wavelength, let's say three, you can use the flux at the longest wavelength to represent red (r), the middle wavelength to represent green (g) and the shortest wavelength to represent blue (b).

Put this together and you have 3 numbers representing a visible rgb signal that can be used to create a picture. Of course there may be lots of fiddling that goes on with the colour balance and the contrast to produce an effective picture, but this is the basic process.