When I first learned about boundaries (sadly, just a few years ago), it helped me to write them out, practice saying them out loud, and then communicate them to the person. It was truly a growth process and once I did it, I think my perspective changed significantly. It was difficult to communicate them and I was literally shaking when I actually stated them but I'm glad I did it.
Here is a link to the webpage where I learned how to set and communicate a boundary. Scroll down to the "Formula for emotionally honest communication" section.
Setting Personal Boundaries - protecting self
The basic format is:
When you . . . . .
I feel . . . . .
I want . . . .
Since I am powerless over you, I will take this action to protect myself if you behave in this way.
So, with the friend example you gave, you could say:
When you tell me how to raise my son,
I feel attacked and resentful.
I want you to stop telling me how to raise my son, and stop insinuating that I am a bad parent.
Since I am powerless over you, if you continue to tell me how to raise my son, I will no longer speak with you (or whatever other action you want to take).
I don't know that you can set a boundary regarding someone else's drinking. Unless the boundary is, when he is drinking, you will leave the house or something like that. Which is your choice but what are you going to do if he is drinking at 11:00 PM on a school night?