First step is getting your dream recall up. If you don’t remember your dreams there’s no point to being lucid in them. It also trains your brain to more aware at night.
Alongside practicing dream recall you make a habit of doing reality checks throughout the day. These are short tests to see whether you’re dreaming or not.
The “dream world” has a few consistent broken parts. One is that your hands will tend to have the wrong number of fingers, another is that numbers and words won’t remain consistent unless they’re specifically important. Yet another is that light switches often won’t work the way you expect.
So examples of reality checks would be to look at your hands throughout the day and count your fingers while asking yourself “Am I dreaming?” Or looking at a digital clock, looking away, then looking back to see if the time is still the same while asking yourself “Is this a dream?” Or turning lights on and off to see if they actually control the light, with that same question.
These are just a couple examples, but the purpose behind them is to create a habit that will repeat within a dream. When it does, and the reality check fails, you’ll know it’s a dream and can become lucid… provided you manage to not get too excited and wake yourself up.