These days, I have multiple WIPs (Works in Progress). I jump from one idea to another as I get bored or as I receive new cards. Some Cubes will require more planning than others.
For example, I want to create a Cube for Halloween that is more than just cards from the original Innistrad and Shadows Over Innistrad. However, it’s difficult to plug away at something I won’t be playing until October.
Another example, an off-hand comment a friend made at the game shop inspired the idea of a “politics” Cube. I started putting it together, then realized the new Conspiracy set will be released in a few weeks. Might as well wait for that before continuing.
I’ve really enjoyed playing Commander Cube, and wanted to do a Tiny Leaders version. Once I’d added in all of the applicable cards, the Cube came in at 1,872 cards. I’ve never played with more than 6 people at a time, so I just don’t need a Cube that big. However, when I used CubeTutor to filter out the commons, I was left with about 800 cards, which is too few. But the winnowing process seems too daunting right now.
Alternatively, some Cubes, like “Make or Break,” come together very quickly.
My point, I suppose, is that there’s no right or wrong way to approach Cube building. Focus on one until it’s finished, or jump around.