Any game idea will never be complete until it has been tested and played over and over and over (and over). In fact, most ideas for a new game are not complete until a lot of time is spent refining it. You may have some new game mechanics figured out but don't know how many spaces the board should have, or you may have a unique board design in mind but don't yet know exactly what all the rules will be for game play. Even a new game concept that seems complete will have a few wrinkles that need to be ironed out. And the only way to do so is to play it, especially with others who aren't as excited about this new idea as you are. It's easy to fall in love with your own ideas, so any honest feedback you can get will either help you mold your concept into a game with broader appeal or it will help you come to realize that maybe your idea isn't so great after all.

The point of all of this is: any new game idea needs to be turned into something that can be played. Draw a board, get some dice, create a set of cards, and make or get whatever else the game needs - a spinner, a timer, playing pieces, whatever. Do not worry about how pretty the game looks or whether you think the board is the correct size, as long as you can play it! Use construction paper, poster board, pieces from other games, coins, buttons, whatever you can find. If you need dice other than the standard six-sided dice that can be found just about anywhere, most game stores carry dice of all kinds, especially if they sell roleplaying games.

And that's it. One important reason you don't want to waste time making anything fancy is that you will likely makes changes as you playtest your game... lots of changes. In fact, you may find yourself making changes even after your game is "perfect" just because you get in the habit of constantly tinkering with it. This is another reason to pester friends and family members to play your game as you work on it - so you don't make too many changes and ruin it once you've achieved the best version it can possibly be.

Lastly, write down all of the rules to your game as completely and thoroughly as possible, making sure that someone other than yourself can understand them without you needing to explain anything. And as you make changes, keep your rules up to date. This may seem like a tedious process, but sloppy game design leads to confusing rules. If you want your game idea to go beyond your own friends and family, this extra work will pay off when you try to pitch your completed game concept to strangers, especially to game agents or directly to game companies. They will make decisions about the originality and marketability of your game based on your rules (and other factors too). No one will expect your artwork or self-made graphics to be perfect, but your rules have to be.