This the second part of How I Was Able To Be Successful Even When Forced To Use Waterfall
Rule #1: take your time
Luckily, your estimation meeting will be much more fortunate than mine (see Part I).
In my previous Scrummerfall experience, since I was forced to produce a big-planning-up-front phase, I was used to always plan 2 or 3 days for it. I was asking for an estimation when my ignorance of the problem was at the maximum level, hence I needed a lot of analysis.