We are renovating an old home, and want to know how to get that "mud" effect on the walls so we can then paint

If you want that splatter effect, they sell a spray gun for this. It has a little hopper that you put the drywall nud into and then just sray it on. I did this in a few rooms and it turned out fine. Start in a closet for practice. By the time you get done with the closet, you'll have the knack of how much to spray on, how close, etc. I bought the sprayer at Home Depot. I think it was about $50. Good luck.