I hear this question a lot both in real life from my students and virtually as people often ask this question in the My English Club Chat. And the honest answer is: You can't - not without reading.
The only way to improve one's reading comprehension is to read. The more you read the better your vocabulary will become and the better your reading skills will develop. There is no tonic to drink or mantra to say that will magically make you a better reader. You have to read, and then read some m