A kit is helpful because it has all the components, but certainly not needed. Fairly accurate measurements are important, so a graduated cylinder or other measuring
device is recommended. Beyond that, all you need is nicotine, PG, VG and flavoring. Finding it all at the right price in the same place can be a little challenging.
And Hoosiers advice on keeping notes is one of the most important things you need to remember. Even if you are just experimenting and
throwing a few things together,
write it down! Some flavors take a few days to be at their best or grow on you and getting to the end of a great bottle and not remembering what you put in sucks, trust me.