You cannot leave an engine idling whilst stopped for any length of time. Parking must be in the direction of the traffic so facing the direction of travel on the right hand side of the road.
There are meters and ticket machines on many streets in Belgian cities. Sometimes you'll find meters in a blue zone. There, the meters override the disk and so payment must be made. There are also many municipal and private parking garages.
It’s possible to buy an International Blue Parking Disc from many shops and kiosks. There are special parking areas for holders of these and you can park there for a set number of hours. It’s often a much cheaper way to park.
Clamping is used in Belgium but for uninsured vehicles, not illegally parked ones. Overstaying your ticket is monitored by police and wardens and you'll get a ticket if caught.
Belgium follows EU guidelines on parking for holders of an EU Disabled Parking Permit but you should check details carefully when parking as some parking areas or car parks make exceptions.