How should I position AC/heat ceiling vents?

They should blow the air towards the wall. You don't want the air to be felt blowing on you directly if you're in the room. By blowing the air that direction you are 'washing the air' against the wall creating a current for the the return register to pull that air back into the system.The best option with any ducted heating and cooling system is to keep your vents open at all times. This allows optimum air flow for the highest possible energy efficiency. If you want to direct air away from a wall or corner, you can adjust the fins so they’re slightly slanted toward the room that you’re in. Avoid closing the vents entirely.Closing vents in your home isn’t an efficient way to manage airflow. Your HVAC system is designed to send a certain amount of air through the house, based on its square footage. It will continue blowing the same amount of air, whether your vents are open or closed. If you close too many vents, air pressure will build up in the ductwork. This causes your HVAC system to work harder than necessary at getting air through to the house.

A room with closed vents and a closed door will also have a pressure difference. It will suck air in around the doors and windows as best it can, diminishing your energy efficiency and possibly impacting your home comfort by creating uneven hot and cold spots.

Lynn is absolutely correct! This is the foundation of duct design (also known as engineering manual D) for an Hvac system

If your vents grilles don’t have multi/position louvers, Replace them with new ones that do

If you replace the vents make sure you have insulation around the edges so air does not leak out.

Place your registers where the air flow is towards the outside walls. In this manner it will help overcome outside air that can make its way through cracks, crevices, windows etc. And especially if you don’t have insulted walls.

Before installing all systems, it is necessary to prepare the surface. In general, according to the technology, they are mandatory during the finishing works of this type by technology. Therefore, people usually order air conditioning installation services. However, they are often ignored, believing that the suspension structure is designed to hide all the flaws. So, first of all, you need to seal any cracks and joints. Naturally, these are cold bridges, provoking condensation. It is for this purpose is also recommended to carry out thermal insulation. This is relevant for residents of private homes and the last floors of apartment buildings with uninsulated attic space.