True Venetian plaster is a bit different than what most folks call "Venetian" plaster. The real stuff is lime based with marble dust or flakes thrown in. This gives it a sparkle as it is not painted.
You can use drywall mud and a trowel to get the rustic look. I have been working on a condo remodel in which we are doing this for all of the ceilings. (we are doing this as a method to encapsulate some very fine "popcorn" texture that contains asbestos)
I like to use a 12" drywall knife for this, it is applied after some mixing in the tray to render a smooth mix. the goal is to apply it in a random fashion so the ripples and edges are going in multiple directions. Your technique should be refined enough to not produce bubbles.
High ridges are gently "knocked" down after completely dry with a sanding screen...form there it is primer then paint.
You can see a little of this work in this pic (the ceiling)
Honestly I would not recommend this as a DIY project. Any type of plastering is labor intensive and is a highly skilled craft. I would recommend two options:
1. Hire a tradesman that does a lot of this type of work to do your job
2. Hire a tradesman that does a lot of this type of work to come and teach you while they do most of the work if you want the experience.
Comment Just my two cents