As you hopefully learned during your time with GameStar Mechanic, a good portion of your time spent as a Game Designer comes from making and playtesting small (and sometimes large) tweaks to your game. Often these take the form of changes to number values or equations once you have your basic mechanics in place.
There's no better place to practice this skill than with existing analog (board, card, etc) games where no one can hold you back from changing the rules however you want. Chances are you've played modifications like this, known as House Rules without knowing it (for example most people play Monopoly according to house rules without knowing it, usually regarding auctions, try reading the rules sometime!)
1) Grab any board or card game you have in your closet, preferably with friends but if not then something solitaire (why not solitaire itself?) or something you can at least pretend to play against yourself (doesn't require hidden information). Don't have any games around? Try modifying rock, paper, scissors (see Rock, Paper, Scissors, Lizard, Spock for example) or Tic-tac-toe (how would having more squares affect the game?).
2) Change 1 rule (numbers are easiest, try doubling or halving a number) and try the game out. It's easiest if the game is short or you can try out the rule with a single round for faster feedback on how the rule change works.
3) Start a blog post analyzing how your change has effected the gameplay. Continue making changes (both to the same rule and possibly to other rules) and recording them and their effects in your post. Super bonus points if you post pictures or video of the modifications in action!
4) Summarize what you've learned about how the kind of changes you were making affect a game and anything else about this topic you learned or thought about.
5) Reply to this mission topic with a link to your blog post and a short blurb about what game you modified, what you tried out, and how it worked out (just enough to interest others in reading your full blog post!).
6) Add "Remixed a game" to your Missions completed.
- Get the Board game remix kit, it's cheap! (available as an e-book, physical book, cards and an iPhone app)
- Check out Cheapass Games. They design games that use many of the pieces you already have lying around from your board games to sell you games really cheap. They now offer a large amount of their games for free!
- Check out some of the free Print and Play games (making these easy to modify!) on Board Game Geek. BGG is also a great place to find alternate rules and remixes for the games you love!