Guineas like to lay on the ground. A piece of plywood leaned against the wall in a secluded area, with some nice bedding to form a hollow in underneath, makes them happy. Site the board so they can exit on either side, too...they like having two escape routes to quit their nest in a hurry...just in case.
In temperate climates, Guineas tend to be seasonal layers, so don't be too disappointed if she doesn't lay for you in the near future. Depending on where you live, her laying may even be finished for this year...you might have to wait until next spring for more eggs!
You will always have the best luck with Guineas if you think of them as quasi-tame ring-necked pheasants rather than chicken wanna-bes and care for them accordingly.