The BBC has a good Q&A on liquid explosives. Here's a key snippet:
How are they made?
There are such things as liquid explosives that are high explosives and they behave in exactly the same way as solid explosives, such as TNT.
But there are also explosives that are made by mixing a solid and a liquid - one being the oxidant and the other being the fuel. Unlike most high explosives, they do not contain the fuel and oxidant in the same molecule but they do contain them in sufficiently close contact to cause a blast.
Are the components difficult to get hold of?
No, it is very easy. Ordinary household substances could be used.
Specialist knowledge or equipment needed to make?
If someone wanted to obtain a solid high explosive in a liquid form, it would not be difficult for a trained chemical technologist.
But if someone was using a backyard laboratory it is more likely they would go for the two component approach.
Not a lot of experience is needed, the principles are quite simple but it would be a hazardous process of trial and error.
I would not want to be messing about these things. It has been known for schoolboys to go home and attempt this and blow their house up.
Could an explosive device be carried on to an aeroplane?
The size of a device necessary could be carried in hand baggage. Explosives in a toilet bag, certainly inside a shoulder bag would be enough to meet the terrorists' needs.
They could be quite hard to detect because I do not think any of the things we have mentioned would respond to x-rays. For example, a liquid hydrocarbon fuel could pass as mineral water.
The question is how do you get something packed into a bag so it does not look suspicious?