Over the years, I have had a few people ask me about my process of songwriting. I certainly do not consider myself an expert, but here are a few thoughts that may help you in your process.
Is there a truth that has been stirring your affections for the Lord lately? If so, write it down. Don't wait, or you may lose it. Is there a melody that you can't stop humming? If so, record it. Do it quickly, or a competing melody will soon drown it out.
If I have my computer nearby, I use GarageBand to record ideas. If not, I use Voice Memos, the audio recorder on the iPhone. Whatever the application, use something to record your creative moment.
When you have something, take the time to immerse yourself in it. When I started writing "To The Cross I Cling", I had this great line stuck in my head from "The Valley of Vision": "All things in me call for my rejection. All things in You plead my acceptance."
I wrote that down and worked in truth around it. I sat and recited that line over and over. As I did, a song spilled out on the page resulting in truth that rhymed and a melody that was so fine. Sorry. So, what's my point? It takes time to make words rhyme and be theologically sublime. Again, sorry.
When you sit down to write songs that teach sound doctrine and stir people's affections for Christ, do not sacrifice content for haste. If you can't find the right words, just wait. You can also ask a friend who loves Jesus and knows the Bible well to enter the writing process with you. I love collaborating on songs with others.
Sometimes a song takes five minutes to write, but that is rare. Most songs take a day, week, month or year, and that's okay.
Make sure you aren't writing for the radio, either. The gift you've been given in songwriting should be used to glorify the One who gave it to you. Always remember the glory of God as you write.
You want it to shine brightly through the song you're writing You want to teach deep and sound doctrine through it. You want those listening to taste and see that He is good.
Take your time. Listen to God. Put to melody what He says.