How do you ask someone who is laying down to go into a seating position ?

For once the answer is exactly what you would expect it to be lmao

"Sit up"

Tell them to sit up!

For someone who is LYING down, you would say, “Sit up, please.”

“Can you sit up?”

Always the direction they’re actually going. So you always stand up and lie down, but you can either sit up or sit down, depending on where you started. 

Sit up. Take a seat. Have a seat on the chair.

The action you're describing is sitting up! As opposed to sitting down (which is from standing). You would say "please sit up"

Can I have you sit up, please?

You don’t say sit down because they’re already lower than you want them to be. So you say sit up.

"Sit" is the correct imperative. "Seat" isn't usually used as a verb, except in specific circumstances like when you're guiding someone to where they will sit. So you would tell them to "sit up".

We say “sit up,” “can you please sit up,” “would you mind sitting up, please,” or “goddamit, sit your ass up,” depending on context. But in general, its “sit up.”

You would tell someone to “sit up/down” based on where a sitting position is from their current position. “Sit down” is basically only useful from a standing position, just about any other position (including slouching) would use “up.”

“Sit up”… example: “Stop slouching, sit up” Past tense “sat up” ….. example: “a noise woke me and I sat up in bed”

If someone is standing up and going into a seated position, that would to "sit down." However, if someone is lying down and going into a seated position, it would be to "sit up." You always use the direction they are going to get into that position. "Sit up" can also be used to tell someone to sit straighter, if they're slouching. It's not uncommon to use with little kids, as they often lie on the floor more than other people.

“Sit up”