If you're a menthol junkie like me, then having menthol drop or koolada is a MUST. You can purchase it premixed at TFA or OneStopDIYShop
here and
here. Both places dissolves methanol crystals using PG or PGA to attain a 10% concentration (i.e., 10g/100ml), but you can also make your own by purchasing methanol crystals on ebay or Amazon and dissolve it yourself. As for how much to add, a good rule of thumb is always start low and move up (I would start with 1 drop per 3ml if using 10% concentration). Menthol crystals/koolada are tasteless, it adds a TH kick and enhances the flavors. Based on my limited experience, menthol drops are best used when adding to minty, fruity, tobacco type flavor ejuices. Here's a thread on ECF on how to DIY menthol drops (you can find more using ECF search engine):
http://www.e-cigarette-forum.com/forum/diy-e-liquid/20590-menthol-drops-recipe-menthol-lovers.html
Good luck and happy vaping!!!
PS For smaller quantities, I would purchase it from WL
here. If you decide to go with WL, you might also want to consider picking up a bottle of EM (ethyl maltol) and marshmallow/caramel/vanilla flavor. EM adds sweetness (burnt sugar) and marshmallow/caramel/vanilla are often used to add depth or as a base for DIY eliquid.