Pumping

I have 4 month old who is ebf.In couple days I have to run some errands but I can’t bring him with me so I will leave him with his aunt.He doesn’t take formula but I want to pump at least couple ounces so he can eat until I get back.

I will be gone from probably 8 am till 11:45 am.

So how do I do it?

Do I pump at same time he is feeding or after and for how long?

And if he nursed on right side and I pumped on left side next time he is nursing should I put him on left side?