There are number of ways you could do this but the easiest is to assemble a modular course using a series of felt-covered boards.
Before you start the project, sketch out your design on a piece of paper. Measure the area where you want to place the mini golf course. The boards will be made from sheets of medium-density fiberboard (MDF) that are sold in precut sizes. Call your local home supply store to learn the available sizes, then decide how many you want to use — and in what configuration — to design your course [source: This Old House].
To raise the board off the ground, you need to glue composite wooden balusters (long, square-sided posts) along the edges. Then you can apply green felt to the board surface using an adhesive spray. To make a hole for the ball, use a 3-inch (7.6-centimeter) hole-cutting attachment on a drill, then insert a small section of 3-inch PVC pipe and insert it into the hole as a "cup" [source: This Old House].
Glue more balusters along the top edges to create walls. The placement of walls depends on where each section fits into your larger configuration. For the corners, cut a short section of composite baluster at 45-degree angles to create bumpers. You could also cover the surface with AstroTurf to produce a slower, more controlled roll [source: This Old House].
One advantage of this design is that the boards can be separated and stowed away in the garage or basement when not in use. But some folks won't settle for anything less than the real deal: grass.
Building your own backyard putting green is an expensive and complicated proposition. Expect to pay several thousand dollars to create a professional-quality green [source: Downs]. Then there's the maintenance. You can't use regular grass for a putting green. You need to seed it with a special variety called creeping bent grass, and keep the bent grass perfectly watered, aerated, fertilized and trimmed [source: University of California]. But think how cool a real sand trap will look!
Another alternative is to use artificial grass on your ground. Nowadays it looks far more realistic than it used to. Artificial grass may cost thousands of dollars to install professionally, but once it's laid down, you don't have to water or mow and it will last for decades [source: Abkowitz].
Now that your foundation is laid, how do you add the obstacles?