By doing it.
The perfect answer to this kind of question does not exist since everyone learns differently (and at different speed) but I do think the best way to learn something quickly is to do simply do it.
Find a good tutorial or article explaining how to build a webapp from the ground, and reproduce the steps yourself as you read it. Once you feel comfortable with the tech/language/framework/whatever you are learning, try to break free from the tutorials and code something yourself. Having a concrete project in mind is also very helpful: you'll feel more motivated to learn something if you know how and why you will use it.
Also: set progressive goals and steps, take your time (learning quickly is possible, but learning too quickly is a bad idea), read a lot of documentation and don't be afraid to ask for help when you need to.
Also be careful of too high expectations, learning how to code can be quick, but nothing beats experience, you'll get better over time and that is something you cannot rush.
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