Though I think this is pretty self explanatory here is what you do:
-choose a highlighter
-colour on a rubber stamp
-stamp your stamp
Easy as pie (which I think is a weird expression because making pie is not that easy... there are a lot of steps involved and this only had 3 which could have even been said in a single sentence if I had wanted to). You can even mix colours by layering to get different shades like the tri-coloured K in the bottom left of the picture below. I even tried doing a bit of ombre-esque mixing... that'll take some more practice to perfect. It's also a perfect and simple way to incorporate the neon trend into your projects. Now I just need to find more things to stamp....