Preparations are unknown since this happened before the show began.
The ceremony is also unknown because Rachel had run away and into Central Perk.
After Rachel ran off from her wedding, she found her best friend Monica Geller and introduced her to Chandler, Phoebe and Joey and re-introduces Ross, Monica's brother to Rachel. Rachel then starts to live with Monica in the latter's apartment. Rachel had ended up running away from her wedding to Barry because she didn't love him and was more turned on by a gravy boat.