You have to find the large springs(Best use Tails) and those will take you to a special stage. Here, collect the amount of rings it says or more(go for more) and then you get to do it again! Yes, twice. You get will continue if the amount of rings needed is gotten. After both rounds, you will get an emerald. I know where 6 of the 7 emeralds are:
#1) Zone 1, Act 1
#2) Zone 2, Act 2
#3) Zone 3, Act 1
#4) Zone 4, Act 1
#5) -Unknown-
#6) Zone 5, Act 2
#7) Zone 6, Act 2

Keep in mind that Knuckles or Tails would be best because all the emeralds are in high places. It is possible to get them with the other characters, yes, whichever you prefer, but I'd choose Knuckles or Tails.

Good luck.

if u jump on the trampoline thingy it shall shoot u in the air and u ride on a skateboard
thing

p.s. i only gots 1 emerald