First reporting: go to the immigration office that your TM30 is registered in.
In theory, subsequent reporting can be done online BUT your first reporting after you enter the country must always be done in person. Since most DTV holders don't seek an extension, they don't need a second 90 day report. When they re-enter after their 180 days, they'll need to do 90 day reporting in person again.
no one really knows since the DTV has been out for less than 5 years, but based on the METV, you'll get stamped in for the full duration even if you enter the day before expiry.
back when visa exempt entries were limited to 2 entries at land borders (before the 60 day visa exempt policy), the government had carved out an exemption for citizens of neighbouring countries (I know about Laos and Malaysia, not sure about Cambodia and Myanmar). Still created issues for expats, though.