Know Your Audience
As you plan your mini-golf course, you must decide whether you want to make a simple course in your house or backyard or whether you wish to attempt to open a genuine miniature golf course as a business. In either case, you must think of who will play on the course. Courses for the general public should maintain a balance between difficulty and ease. No one wants to play a hole with no obstacles. However, holes that reasonably could be considered par-10s will discourage all but the most adventurous golfers.
If the only people will play your course are you and a few close friends and family, think of your dream course. Write a list of everything you ever wanted to put on a miniature golf course. As long as you have the resources, turn your dreams into a reality -- but plan ahead so you don't run out of room.
Pick Your Obstacles and Theme
Plan your obstacles before you begin to construct your course. You might want a singing robot on the middle of a hole, but without planning, you might buy all the components and begin building, only to find that the robot covers the entire hole. Not only will an obstacle list help with logistics and economic considerations, but it ensures that your holes don't become repetitive. Vary your obstacles so each hole thrills your players, especially if you hope to open the course for business.
As you envision your entire course, think of it as a whole, not as many holes. For instance, don't go for a scenic naturalistic tropical approach for the first 12 holes, then place goofy figures and obstacles on the next two holes, then return to the natural approach. You want your course to work as a concept as well as a playable course.
Observe Existing Courses
The only way to make the best course possible is to know what already exists. Play as many courses in your area as you can. Note your favorite holes and the ones which work the least. If you wish to open your course for business, talk to some of the various courses' patrons and see which holes they love or hate the most, and observe how other people react to these holes. As you note the pros and cons of existing courses, think of what unique aspect you want to bring to the world of courses. If your course will only cater to you and your family, consider your own interests. What movies, music or vacation spots do you like the best? Model your course after some of you and your friends/family's favorite things. For courses open to the general public, choose a theme that other courses have not captured yet. If all the other courses in town are naturalistic, consider a wacky course full of volcanoes, animals and other off-the-wall obstacles. Yet if all the local courses rely on gimmicky obstacles, opt for a "pure" mini golf course that uses real greens, with sand and water as hazards.