Hello! I just wanted to make this post after being inspired by @mooseontheloose 's and @delda 's posts (though unfortunately Singapore is small)- if there are any inaccuracies or queries feel free to let me know!
- What postmarks are there?
You can check @sgstampers (ig), SG Stampers (FB) etc for regular updates on new postmarks. Normally they’re in Miniature World, a free magazine that Singpost releases quarterly to announce upcoming issues. Rule of thumb is that if there are images of the stamp design and stuff like FDC themselves (See Image 1) the issue is confirmed. If it is just a name at the back of the issue (See Image 2) for a future release, it is not confirmed, and sometimes it may not happen.
(Image 1)
(Image 2)
- Where do I get these postmarks?
You can head to the Philatelic Shop at General Post Office (Singpost Centre) or the one at [Fullerton Hotel](Philatelic Store @ Fullerton 6223 1337 https://maps.app.goo.gl/DaCXP4WdccwG5TFNA) (B1). In my experience, the one at the General Post Office allows you to stamp the postmarks after the launch date so as long it is before the launch date of the next stamp (or sometimes even after, if the two stamp issues are released very close to each other). The one at Fullerton Hotel does not allow you to stamp the issue’s postmark (even if you have no wish to send it out i.e. stamp and postmark in front side of postcard) a few days after the stamp issue in my experience, but I suspect it might be up to the staff manning it.
- How to stamp?
Firstly, when you visit the shop, you buy the relevant stamps and make payment. Then, after you pay, ask for the ‘chop/postmark’ - below is an example (Image 3). (You could also try asking for the postmark chop without buying stamps, but i’m not sure if they’ll indulge your request.)
Next, you will get the chops (yes, fully manual; no automatic chopping unless the ones in Hong Kong or something), with an inkpad and a stack of paper. Before you stamp onto your card, stamp onto the paper as a test first. You might take a few tries so dont worry!
After testing, you can stick stamps onto the card (Image 4), before putting the postmarks onto the stamp and card. (Image 5) And that’s it! Take note that Singpost DOES NOT allow you to mail in for a postmark - you have to come here yourself (or get a Singaporean postcrosser?)
(Image 4)
(Image 5)
Final note: If you’ll like to swap with me, feel free to let me know! I’ll probably be posting by tonight on the swap thread so you can approach me there 
If there’s anything that I missed out feel free to let me know too!