Yay, we have finally collected the key to our BTO HDB after waiting for around 4 years!
Let me share what was the process like and the things we did to prepare for it!

1. When the time is right, you will receive an SMS/email from HDB.

SMS came first

2. Logged on to HDB to retrieve your Letter of Appointment. It looks something like this.
Under normal circumstances, one would not need to prepare so much cash on the appointment date. Ours was so high because we applied for the BTO many years back when we were on a single income. CZM was working while I was still in the university. As a result, the HLE we have gotten then (~$200k) was not enough to loan/cover the remaining amount (~$400k). Anyway, a few weeks before the scheduled appointment, you should have received an email/letter asking to review/update the HLE....