Hi celine, sunnysideup,
I assume you both are planning to solemnize outside ROM building and not planning to solemnize in a church cos there are separate considerations for those.
You will need to decide on the exact timing (for e-filing and to tell your chosen solemnizer) and date of solmenization. Then choose the lunch venue and book it.
You will need to contact a solemnizer (contact details are on ROM website) to obtain his/her confirmation that can solemnize for you.
If you took up a bridal package for your AD, you can check if that package comes with a ROM package. If it doesn't, you will need to source for your own make up artist, and rent or buy your own dress. Daniel Yen has nice suitable dresses. Alternatively, some bridal shops do sell/rent solemnization dresses too. If you want a photographer you will need to book one too.
You will need to also e-file on ROM website.