There are numerous threads on these very questions, one such resource is the Sodium Nitrite Megathread, linked below.
A quick answer to your questions:
Thats up to you. Many members measure out the correct dose and mix with just enough lukewarm water until it is dissolved. I dont reccomend you dilute it too much as SN is an oxidizer.
Do I need any other medication to not throw up.
That is completely up to you. Many members have either gone in person to their physician, or used telemedicine providers to get a prescription for an antiemetic. It seems like Metoclopramide is the best tolerated antiemetic based on forum talk, but not all doctors will prescribe this particular med.
How much SN should be mixed with water.
refer to the megathread, Stan's Guide, or the PPeH.
https://sanctioned-suicide.net/threads/sn-resource-page.32633/
https://sanctioned-suicide.net/threads/sn-megathread.1156
https://sanctioned-suicide.net/threads/pph-april-2022.89185/
https://sanctioned-suicide.net/threads/stans-guide-to-sn.27535/