Well - it's a fairly striaghtforward process. E-liquid is simply Propylene Glycol, Vegetable Glycerin, nicotine and flavorings. The exact ratio of these ingredients varies according to what properties and flavors you want the finished liquid to have.
The raw ingredients are easily available and you will need some minimal mixing and measuring gear.
The big issue is often in trying to recreate a favorite commercial liquid - it can often be a frustrating task.
Try reading a few of the stickies in the
DIY sub-forum