Plan at least 3-4 months ahead
1) Before Registration
Invite two offical witnesses, and get their photocopy of ID
2) Registration
1) ROM on weekday (Mon-Fri) office hours: register at venue of ROM. Pls check ROM website for details www.rom.gov.sg.
2) ROM on weekday off-office hours or weekends or public holidays: find a solemniser from ROM website, contact him/her and agree on a venue and date and time where you will have your ROM. (Note certain details can be changed later, e.g. solemnier)

After registration, they’ll give you a VD/SD appointment which you and your partner bring necessary documents to ROM and collect the blank Marriage certificate for use on ROM day.

3) Preparation
Once the above is confirmed, prepare the following list:
1) Book stylist for makeup and hair-do (OPTIONAL)
2) Dress (rent or purchase) (UP TO INDIVIDUAL COUPLE)
3) Photography and video (you can ask friends to help or engage professionals) (OPTIONAL)
4) Rings (OPTIONAL, and UP TO INDIVIDUAL COUPLE, and remember to let the solemniser know in advance if you do not bring rings into the process.)
5) Accessories: shoes and other accessories for bride, flower bouquet for bride, etc. (OPTIONAL)
6) Guests (family and friends) you would like to invite to witness the solemnization (up to 20 ppl at ROM and if outside ROM, depending on the number of ppl can fit in the venue you register previsouly) (OPTIONAL)
7) Celebration (lunch or dinner or high tea with family/friends. After you confirm the ROM date, time and venue, you can invite your guests and confirm the number of ppl attending and then book with restaurants/hotels and confirm the menu etc.) (OPTIONAL)

4) VD/SD Appointment
Collect the documents at ROM and remind your solemniser if you book outside ROM and remind the two offcial witnesses.

5) ROM DAY
1) Do not be late. So plan the preparation to be completed with 1-2 hours buffer time.
2) Check out the venue several days before your actual day for travel, navigation and parking if outside ROM
3) Remember to give hongbao to solemniser after the process, for reimbursement of transportation cost (recommended $48 on ROM website)

6) After ROM
1) Update your status of “being married” for certain organizations (e.g. your companies, insurance companies) (UP TO INDIVIDUALS)
2) Thank solemniser, two witnesses and other ppl who served and helped and ppl who witnessed your solemnization and ppl who attended your celebration (UP TO INDIVIDUALS)
3) Returned rented items if any
4) Collect photos and videos if any and distribute to friends and family who request for
5) Plan for your wedding

Based on my own experience.
hope it helps :)
Please also correct if necessary so that others can get the right information.
cheers,