All you do is go to any world and search from one end to another then stop & drop, do the same with the next character until you come across a Special Portal you've yet to tackle. (I usually do a clean sweep of a world twice before moving on to the next).
Would be a good idea to write down what ones you've beaten and in what area, just for future reference... because this game has the knack of making already-beaten Special Portals re-appear
All you do is go to any world and search from one end to another then stop & drop, do the same with the next character until you come across a Special Portal you've yet to tackle. (I usually do a clean sweep of a world twice before moving on to the next).
Would be a good idea to write down what ones you've beaten and in what area, just for future reference... because this game has the knack of making already-beaten Special Portals re-appear