Step 1: Disclaimer
Caffeine is a deadly poison, do not exceed a safe dose unless you are prepared to experience shaking, feeling like you are going to die, and/or actually dying. When handling it take appropriate safety precautions and wash your hands afterwards. DO NOT LICK YOUR FINGERS AFTER HANDLING CAFFEINE.
If under 18 undertake this experiment only under the supervision of an intelligent, competent adult.
I ACCEPT NO LIABILITY FOR ANY ACCIDENTS THAT ARISE AS A RESULT OF PERFORMING THIS PROCEDURE.
That said, if you follow the instructions carefully and apply common sense this is a fun, safe and rewarding experiment.
Step 2: 'Ingredients'
A high precision scale (one that reads to .01 of a gram)
Citric acid (or another food grade acid)
A disposable pipette (not strictly needed)
All of these are just a google search away.
Step 3: Weighing Out the Caffeine
Start by deciding how much coffee you want to supercaffeinate; I went with 30 grams. Once this is weighed out you need to decide how strong you want your coffee. In the parlance of those who make concentrated extracts of natural materials, strength is often denoted by the x notation. This tells you how many times stronger it is than the feedstock; so normal coffee would be 1x, decaf 0x, and triply caffeinated coffee would be 3x.
The internet (http://www.coffee-tea.co.uk/caffeine-content.php) claims the average caffeine content of coffee to be 13.5 milligrams per gram. Times this by the number of grams of coffee you have, in my case 30, to know how much caffeine is in it. 30 x 13.5 = 405mg. Now decide what x value you desire; I would recommend no higher than 3, take one from this to account for the caffeine already in the coffee, and times this by the result of the previous sum. (3 - 1) x 405 = 810. This is the number of milligrams of caffeine you need to weigh out.
DO NOT ATTEMPT TO EYEBALL THIS VALUE OR USE A SCALE WITH A PRECISION LESS THAN 10mg. THIS COULD RESULT IN FATAL OVERDOSE.
Step 4: Dissolving the Caffeine
Basic nitrogen atoms can bind to the hydrogen ions in acids to form what are known as salts. In its native state caffeine is only very slightly soluble in water, as a salt it is much more so. The salt I've selected is caffeine citrate, because the required acid - citric acid - is safe, cheap, and solid (which makes it easy to measure out) but you could substitute in other acids like white vinegar if you're confident with the maths.
The solubility of caffeine citrate is 95g/litre and it is made from roughly equal masses of caffeine and citric acid. This means for every gram of caffeine you need around one gram of citric acid and 20ml of water (preferably distilled) - you can measure this out with the pipette. Mix the three, heat in the microwave on full for around five seconds, stir again, and heat some more if necessary. This should give a nice clear solution.
Step 5: And It All Comes Together!
UNEVENLY DISTRIBUTED CAFFEINE COULD MEAN YOU GET A HIGHER DOSE THAN YOU BARGAINED FOR. THIS RISK IS AMPLIFIED FOR LARGER BATCHES - SMALLER IS BETTER.