Pick a word that has meaning for you. Classics are: ohm, Rama, and Christian prayers "Mother Mary, etc."
Sit relaxed and with each relaxed breath, breathe your mantra. When you find yourself forcing the word or playing with the word or your mind drifting so you're just listing your mantra (all this will happen) gently return.
Remember: There's no right way or wrong way to meditate! You can't do it wrong! It's a tool to help still the mind so all those lovely creative ideas can come.