Disclaimer: Jesus didn't journal. Paul's letters could be considered journaling, I suppose. Ultimately, writing thoughts in a book don't make you more like Jesus or more/less of a Christian.
Now that we have that out of the way...
Since 1998 I've found a journal to be a comfortable thing, much like that old worn chair you've had since you can't remember - you know, the one that smells like old dust and food stains - it's a place where you know what to expect when you sit down. It's where you rest after long days, where you sit to have great/uncomfortable conversations, where you land after a great loss, where you sleep when things haven't gone as you would have hoped.
The journal has been a place where I've wrestled with demons - both those in flesh and those in my head - but it has also been a place of breakthrough and hope.
Where I've figured out how to forgive. Where I've admitted, committed, and submitted myself in profound ways to God. Where I've realized I wasn't as holy as I thought I was, and consequently realized how much more God wanted for me than I wanted for myself.
My strategy for journaling for the better part of 14 years has followed a very simple structure, and I'm offering this to you with the disclaimer that I don't believe everyone should do it this way.
1. Write to clear your head, in a way that makes sense in your head. I tend to write as if someone is going to read it, because they may, but also because I'm used to writing for audiences. Even if my audience is simply God and myself, I write in a loose and detail-report style. I try to clear out my head by talking through issues that are going on or feelings that I'm noticing, and they often form my prayers for the day.
2. Write your thoughts about Scripture. I typically like to write about what I'm reading in Scripture or in whatever book I'm working through. I write notes and quotes, but also reflections and questions that come about as a result. I'm not picky about whether or not I write the "right" thing, but I write my impressions and questions in the moment and come back to reflect on them later. Again, these often plant themselves as seeds in my mind that grow into plans later in the day or at some point in the future.
3. Write nothing that you wouldn't say/haven't said to your spouse. I actually torched one of my earlier journals because it contained thoughts and ideas that I should have been bringing to my wife but instead was exorcising in my journal and never dealing with in my relationship with my wife. Some may disagree with me on this, but if you want a deep and spiritually sound marriage you can't let a private journal hide what God may want to bring to light in your relationship.
4. Actively reflect on your previous entries. God is weaving a complex narrative with our lives - one of hope and fear, joy and trial, rising and falling - and it is so easy to lose ourselves in the plot and get caught in the fog of every day life. I review regularly over my journals and since I keep them I have the opportunity to do a "last year at this time..." review. It is powerful to see either how God has answered prayer or how I've not released something that is killing me to hang onto.
Again, I'm a journaling-person. My spiritual pathway is "contemplative" so I have no problem being inside my head and getting things from that chaotic place and putting them on the page for closer inspection. However, if you engage in the practice of journaling in a way that makes sense to your brain - I believe the Spirit of God will help you sort out some of the loose ends you may be struggling through today.