I've only ever used water, I actually avoid spraying it with soap because I don't want it getting sucked into the fabric. I don't know if this is the best way, but it's what I do!
Soap is alright, use a brush not a mit because the mit will leave stuff behind, I wouldnt use soap too often, there is proper cleaner you can buy for it. I would imagine soap would dry it out and cause it to fade and crack over time even though people say its fine.
i just did mine about 4 days ago. I brought some vert top cleaning from the auto store.
You have to wet the top than spray some cleaner on a soft cloth and rub very hard. Than you get a dry towel (turkish towel) and rub going back to forward...