The new show follows the pop bands from more than a decade ago as they are put through their paces during two weeks of intense rehearsals, before a one-off reunion performance.
The show will be screened on January 31, and will also feature Atomic Kitten, 5ive and Liberty X.
The nerves were showing for the Dublin girl group, who admitted their rehearsals have been "up and down" so far.
"We're nervous about everything. It's just pulling it off, we have to pull it off," said singer Edele.
The group, whose first four singles topped the UK charts, talked about the new challenges the show presents.
Lindsay said: "When we were a band, we had our rehearsal period in private, then we put on a finished product and people just saw that, whereas now they're going to get to see everything that goes on."
Edele's sister Keavy joked: "The good, the bad and the ugly."