After finishing episode 2, go to your sticker book and head back to the playground. In the first park area where you saved Muffin there will be a dark blue seesaw. Simply jump on one end and the achievement should pop, if not you can hold A so it springs you up but it should pop way before that.
The red seesaw in the other area won't work for this.
If you need a little help with this you can use my video guide below which will start at this point: