For the purpose of crediting the hourly Stand achievement, your Apple Watch aims to identify when you have stood up and moved around for at least one minute during that hour.
Apple Watch uses an accelerometer to identify movement, so arm movement equivalent to changing from a seated to a standing position followed by a minute's walk around with your arms swinging naturally should help to register this.
Credit may be earned when seated if your arm movements are misinterpreted.
It may help to calibrate your Apple Watch, including resetting any existing calibration data.
Resetting your calibration data and following the instructions for calibrating your Apple Watch can be equally as helpful for Series 2 models and will not erase your Activity history.
Activity-related estimations also depend, in part, on your personal information. To check that this is accurate - and to update it over time:
On your iPhone, in the Watch app, go to: My Watch (tab) > Health > Edit - tap and adjust items, then tap on Done.