How can I get my child used to not being held all the time?

Schedule it. I find it rather easy to get babies to accept alone time if you let it happen naturally. Feed as they wake, follow with body maintenance, follow with play time until tired & put them down alone while awake so the develop self settling as a habit. Do not feed before sleep. Do this before 6m or they enter the period of separation anxiety before having any self settling skills & you are stuck.
Tantrums. You have to make sure that your baby learns how to be on his own let him cry a little won't hurt it all depends on you.