In that case, you need the right tools!
A fellow WIS had a little dust under the crystal of one of his HMT watches, and asked me if I knew anything about how to fix it.
I have opened a HMT watch before to clean the inside of the crystal, so I sort of know the process. Below is a crude step by step process on how I went about doing it.
Note 1: I am in no way an expert regarding this topic and I do this only with my cheaper, beater watches. So anyone trying this, should do so at their own risk
Note 2: I have a press fit tool to snap the case back shut, but I've been successful with just my bare hands in the past. However, while using your hands, be very careful not to put pressure on the crystal, rather than the case.
I apologise, in advance, to any experienced watchmakers/repairers in advance, if I'm doing it incorrectly and would love to have any advice from them, on how to do it right.