I have done a novena to St. Joseph before (for similar intentions as you, only to be a good husband and father), here is what I did, and I think you can fruitfully do the same for St. Monica, if you so choose (and from what you say, it sounds like she may be a good choice!):
- Google “St. Monica novena”, and you will find a number of different novenas online.
- Read through them and find the one that ‘fits’ for you. Some of them may put too much emphasis on one or another aspect of the saint (e.g. St. Monica’s dealing with an abusive spouse) that may not apply to you. So look around a bit until you find one whose prayers seem to fit your own.
- Print out that novena, and, when you are ready, begin!
That’s what I did, and it worked well. It’s also quite alright to modify the prayers if you like, or feel inspired to, in order to personalise the devotion. Although in my opinion all that should be done before you actually begin praying the novena, so that once you’ve begun you concentrate on the prayers, and not on tweaking them!
Finding an image of St. Monica, if you don’t have one, to use when praying the prayers can also be useful. We are blessed to be living in the internet age when it comes to things like that!
Other novenas that I have been interested in, and heard good things about, but not yet tried are: Bl. John Henry Newman (whom I have heard is effective in helping people find jobs), and of course Our Lady (I have a particular devotion to Our Lady of Sorrows). I have also entertained the idea of one day composing a novena to St. Bruno, perhaps for increase in the virtue of temperance, but I have not yet done so.
Oh, and I just remembered, I have also prayed a novena before to St. Raphael, who is also a good intercessor for a good marriage! (As well as for healing, blindness, and travel.)