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 cow family, which then ends the stage. You can therefore roll up any small cow 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 Holy Cow, which is a giant cow that hangs around a small forest with smaller cows. You must be 7m8cm8mm 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 cow items in the process.
Here is my run where I collected the Holy Cow, which isn't needed for this achievement but is needed for the King of Katamari achievement: