Welcome to BoundAnna,
Boundi.
You can fasten your belt to the size of your hips together with both arms and 2 inch wider. So take the longest belt for this.
Place it with the iron belt buckle down on your bed where your hips come when you lie down. Afterwards - see below - your back will lie on the belt and not on the buckle.
Sit down on the bed before the belt.
Tie your ankles with the other belt. Keep a 1 inch gap between the belt and your ankles.
Then slowly lie down on the bed while putting your arms in the opening of the first belt.
Make sure that the backs of your arms are turned outwards so that the inside of your arms press against the side of your body.
If you are now completely on your back, your weight will be completely on the belt, which will block your arms.
You can free yourself simply by getting back up and taking your arms out of the belt.
But that way you already have a first experience of what it feels like to be completely tied up.
If you don't have a cold - risk of nasal congestion - you can use the tape to cover your mouth.
Afterwards you can blindfold yourself with a silk scarf (or kitchen towel) and repeat the above to become aware of this too.
You may set an alarm that will expire after 10 minutes and then try to lie down until the alarm calls you back to reality.

Once you have purchased a pair of emergy room safety scissors (these will come in handy if you want to work with rope later), you can use your zipties to cinch the belt around your ankles just as your body now cinch the top belt between your arms.