You should definitely relax before the henna application. Henna coats the strands of the hair and if applied before the relaxer, you would not get the hair as straight. I would recommend you apply the henna maybe a week after your relaxer - someone else could probably tell you better how to space the applications. Also, after the henna be sure to follow with a rich, moisturizing conditioner. I am sure you already do the same with your relaxer.
I apply henna every 4 - 5 weeks.
Nina