How to knock out a person instead of killing them

You will lose more points if you kill the non-target person as opposed to simply subduing them. As outlined in my answer here, you can gain those points back by not being seen and hiding the body.

If you're wondering about whether or not they will eventually wake up from being knocked out (as in previous Hitman games), I have neither experienced this in-game nor have I seen anyone comment on the fact that they do.

A usual and practical way to restore Instinct is by finding a nearby NPC to kill/knockout silently and then hiding their bodies in a container. Check if there's a container nearby where you can safely hide a body before going in for the kill/knockout. Make sure the kill/knockout is silent.

You can also gain Instinct by "hiding". This can be done by pressing the "hide" button when a prompt comes up near an object where you can hide.
^{(Thanks to Ben for mentioning.)}
If you are in a position where you can collect evidence or achieve a mission objective, then you might want to go for those instead, as doing those will also restore Instinct.

To prevent Instinct from going down, avoid being noticed by NPCs wearing the same clothes as your disguise. Sneak/crouch whenever possible when around these NPCs; this will lessen the need for instinct mode.