Constellation stages will become available as you progress through the Make a Star stages.
This constellation stage is different than the others. Your goal is simply to collect any item in the bear family, which then ends the stage. You can therefore roll up any small bear item immediately to end the stage and unlock the achievement. Honestly, the first time you attempt this stage, this is likely what will happen to you.
The ultimate goal, though, is to collect the Kintaro-Bear, which is a bear with a human rider doing laps around the main roadway. You must be 4m62cm or larger in order to roll it up. It's tricky to get big enough, though, because you have to take care to avoid all bear items in the process.
Here is my run where I collected the Kintaro-Bear, which isn't needed for this achievement but is needed for the King of Katamari achievement: