Now that you've condensed your ideas, it's time to build. This doesn't have to be an object.
make your idea physical
A lot of ideas involve apps, technology, and screens. Don't code your app yet! A quick and easy way to prototype is to start with paper.
Print a bunch of iPhone screen outlines. Draw what each screen would look like in the app. Then, hand the paper to a user, and have them interact with the screens as if it was digital. You'll see what's non-intuitive with your interface. Make notes. Then, draw a new one and repeat!
What to Prototype?
Prototyping is all about testing out an idea.
What do you want to test for?
Airbnb's founder wanted to test whether people would be willing to stay in a stranger's house.
There was an upcoming conference near him, but all the hotels were sold out. So, he and his roommate made a website advertising three airbeds in their apartment available just for conference-goers.
They ended up successfully renting out part of their space to three people.
Create a comic-book style storyline for how your idea works. You don't have to be an artist; this is a low-resolution prototype.
LOW vs. HIGH RESOLUTION
Start low-res! Get your idea out of your head and into some bare-bones physical form.
High-res prototypes are for when you've tested multiple facets of your idea. Combine them into a nicer model, and then test it again.