Lucid Dreaming and The Dream Journal.
A Dream Journal is actually quite simple to use.
First of all you need some material, i.e. Dreams.
After weeks or months, depending on your dream activity and the ability to remember your dreams, you should have sufficient material to begin analyzing your dream-material.
Next step is to look for patterns. All people have different patterns that occur in the majority of their dreams. One pattern could be "action". Some people have dreams that are action-packed and very vivid. Others have dreams that are very realistic, while others have dreams that are no near realistic. Each have their own dream-pattern.
You want to find these patterns through studying your dreams. Get to know these patterns as your own back pocket. In time and with practice, these patterns will become lucid-dream-triggers. When you know them very well and they occur in a dream of yours, you will realize them and become aware/lucid in the dream.
That step might take a while so don't become discouraged at first. Every morning when you wake up, write down as much as you remember from your dreams. Have a pen and a journal at your bed table or close by. When you wake up, dont move. Stay frozen for some minutes and lie with closed eyes. That will make it easier for you to remember more details from your dreams.
The great thing about this is that the more you write down, the more details you can remember from your dreams. Soon you will remember 2 dreams, then 3, then 4-5 or 7 dreams when you wake up in the morning. When you have enough material, then begin analyzing.
Do yourself a favor and have at least some weeks of material. That will make it easier to find patterns the dreams. When you find patterns, write them down separately and get to know them. They will be unique, just like a fingerprint.
Lucid Dreaming and the relation to Buddhist Practice.
How can this then be used in Buddhist practice?
In Theravada Buddhism, one can use the dream-state to deepen ones practice.
If one is practicing the WILD technique (see also Tibetan Methods), i.e. entering the dream-state from waking-state, one can continously observe conditioned phenomena as they arise and pass away. Why is that important?
Because the stages before falling asleep, exactly mirrors the stages of dying. There is the possibility to gain some deep insights here. Also to sneak-preview and prepare for the moment of Death. Meditating while going from waking-state into the dream-state gives the opportunity to continue to practice even while the body sleeps.
Let me end with a quote from Tibetan Buddhism, by Shugchang.
"In order to make the time we spend dreaming more meaningful, we must first recognize that we are dreaming. That is the initial exercise. The next step is called transforming the dream; the third is known as multiplying. The fourth practice is to unify the dream with the clear light. Recognizing, transforming, multiplying and unifying the dream with the luminosity of the true nature; these four outline the essential applications of dream yoga."