The first thing I'd suggest you do is understand why you want to do a side project at all. Unless you have a specific goal you want to achieve and you actually value that goal, you'll have a hard time convincing yourself to do the work you need to do to make it happen.
You also shouldn't fool yourself about the amount of time and effort it takes to achieve anything real. Developers tend to underestimate the work involved in achieving a result. For commercial projects that sort of estimation problem turns into cost overruns and missed deadlines. For side projects (without formal schedules) it turns into demoralization when the result you want doesn't meet the timeline of your dreams. That solitaire game may seem like an easy thing to do, but for every complexity you see there's a dozen you don't, and you'll have do the work to solve them all to produce something of value. So make sure it's something you actually care about.
You also shouldn't fool yourself about the amount of time and effort it takes to achieve anything real. Developers tend to underestimate the work involved in achieving a result. For commercial projects that sort of estimation problem turns into cost overruns and missed deadlines. For side projects (without formal schedules) it turns into demoralization when the result you want doesn't meet the timeline of your dreams. That solitaire game may seem like an easy thing to do, but for every complexity you see there's a dozen you don't, and you'll have do the work to solve them all to produce something of value. So make sure it's something you actually care about.