I'm assuming this is related to nursing or medical procedures, rather than servicing people too lazy to buckle their own belts. This means you have some latitude with positioning your patient.

Start with your first step: thread the belt through the belt loops.

Pull the pants onto the patient. Holding the pants in place, get the patient to lie down. To help keep the pants roughly in place, you might have to get them to sit first, then lay down. The lying-down position is much better than the seated position because it avoids tummy action (the tummy tends to push out when the person is seated, making it hard to secure the buckle) and helps avoid unintended lower-than-tummy action.

Move to the person's side and help them buckle up.

I'm assuming this is related to nursing or medical procedures, rather than servicing people too lazy to buckle their own belts. This means you have some latitude with positioning your patient.

Start with your first step: thread the belt through the belt loops.

Pull the pants onto the patient. Holding the pants in place, get the patient to lie down. To help keep the pants roughly in place, you might have to get them to sit first, then lay down. The lying-down position is much better than the seated position because it avoids tummy action (the tummy tends to push out when the person is seated, making it hard to secure the buckle) and helps avoid unintended lower-than-tummy action.

Note: if the patient finds it difficult to move between standing and lying down, you may have to pull their pants on while they are lying down.

Move to the person's side and help them buckle up.