Here is a basic guide for your sermon: http://www.sanctamissa.org/en/resources/books-1962/rituale-romanum/66-matrimony-instruction.html
A few tips:
Keep it simple.
Keep your focus.
Keep it short.
Get your material from the Catechism of the Council of Trent on marriage.
Take advantage of the teaching moment.
Remember that some or much of your audience has not darkened the door of a church in many years, if ever. Gauge your audience.
Address your sermon to the married couple. Stay on topic. Do not include topics that have nothing to do with love or marriage. Do not rant.