The profile page can be found under the 'Me' tab in the main game menu. There are eight separate parts to the profile that you need to change:
1) My Photo - simply select any sticker to display in place of your current avatar. When you enter the profile page, this is the first option that will be highlighted.
2) About Me Quote - make this anything you want. Mine says "boo" haha.
3) Recent photos - snap a picture in any story/community level (start > camera > L1) and upload it from the 'stickers & decorations' section in your Popit (press R1 twice) and it should appear here. So long as the picture was taken inside a level, you will also unlock:
4) Recent Comments - just type your own here.
5) Last Hearted Creation - complete a level and press the heart symbol next your avatar.
6) Last Hearted Creator - to heart a creator, you want to visit their planet by hitting square as you highlight their level. Once the planet loads, hit square again and scroll down once so their name is highlighted - hit X to go to their player profile and heart them here.
7) Latest Review - complete a level and hit the review button, next to the heart / your avatar. Can be one word. If you play a level from the 'Newest' selection, chances are it hasn't been played 5 times yet so by leaving a review here you will also unlock:
8) Recent activity - play any level online for it to appear in this section. Re-play it once it appears for an additional trophy: