Making a quantity of TATP sufficient to bring down an airplane is not quite as simple as ducking into the toilet and mixing two harmless liquids together.
First, you've got to get adequately concentrated hydrogen peroxide. This is hard to come by, so a large quantity of the three per cent solution sold in pharmacies might have to be concentrated by boiling off the water. Only this is risky, and can lead to mission failure by means of burning down your makeshift lab before a single infidel has been harmed.
But let's assume that you can obtain it in the required concentration, or cook it from a dilute solution without ruining your operation. Fine. The remaining ingredients, acetone and sulfuric acid, are far easier to obtain, and we can assume that you've got them on hand.
Now for the fun part. Take your hydrogen peroxide, acetone, and sulfuric acid, measure them very carefully, and put them into drinks bottles for convenient smuggling onto a plane. It's all right to mix the peroxide and acetone in one container, so long as it remains cool. Don't forget to bring several frozen gel-packs (preferably in a Styrofoam chiller deceptively marked "perishable foods"), a thermometer, a large beaker, a stirring rod, and a medicine dropper. You're going to need them.
It's best to fly first class and order Champagne. The bucket full of ice water, which the airline ought to supply, might possibly be adequate - especially if you have those cold gel-packs handy to supplement the ice, and the Styrofoam chiller handy for insulation - to get you through the cookery without starting a fire in the lavvie.
Once the plane is over the ocean, very discreetly bring all of your gear into the toilet. You might need to make several trips to avoid drawing attention. Once your kit is in place, put a beaker containing the peroxide / acetone mixture into the ice water bath (Champagne bucket), and start adding the acid, drop by drop, while stirring constantly. Watch the reaction temperature carefully. The mixture will heat, and if it gets too hot, you'll end up with a weak explosive. In fact, if it gets really hot, you'll get a premature explosion possibly sufficient to kill you, but probably no one else.
After a few hours - assuming, by some miracle, that the fumes haven't overcome you or alerted passengers or the flight crew to your activities - you'll have a quantity of TATP with which to carry out your mission. Now all you need to do is dry it for an hour or two.
The genius of this scheme is that TATP is relatively easy to detonate. But you must make enough of it to crash the plane, and you must make it with care to assure potency. One needs quality stuff to commit "mass murder on an unimaginable scale," as Deputy Police Commissioner Paul Stephenson put it. While it's true that a slapdash concoction will explode, it's unlikely to do more than blow out a few windows. At best, an infidel or two might be killed by the blast, and one or two others by flying debris as the cabin suddenly depressurizes, but that's about all you're likely to manage under the most favorable conditions possible.