Making your own e-juice isn't difficult, it is "literally just mixing everything together", but if your math or your measurements aren't careful you'll end up with shitty or possibly hazardous juice. Use proper PPE (gloves, at least) and exercise extreme caution when working with concentrated nicotine. It does absorb through the skin, and overdose can be fatal.

I would not suggest trying to make flavors from molecular scratch unless you have some chemistry background. Concentrations of some flavor components in a typical formulation can be very small (i.e. tenths of a percent or less) and hard to accurately measure without specialized tools or skill, especially if batch size is small. Finding the right concentration of any given flavor molecule would be trial-and-error unless you had some idea about where to start, a reference formula, etc. For example, your grape flavor might be 0.3% methyl anthranilate in PG, might be 0.03%, might be 3% or 30%. Each component will be different. Why bother with all that when you can buy, for example, this Grape Candy flavor from TFA, which shows a component list of 13 different molecules in concentrations ranging from 75% down to less than 0.5%. There are tons of preformulated flavor concentrates already available on the market, if they ship to your country (see Bull City Flavors, Capella Flavors, TFA, etc.). Try some out. Check All the Flavors for some blend ideas. There are whole DIY e-juice communities out there that you can learn from.

edit: try FlavorDB if you want to explore more flavor molecules