We first took off the bottom six to eight inches of siding and sheathing. We cleared out the mouse nests and other debris from the old stud bays. We scarfed in some new wood in a few places where there was partially rotten sill. Put PT sheathing over that. Drilled a two-inch hole at the top of each floor's stud bay. Blew in cellulose until it wouldn't blow in any more. This requires a special blower, not the one they rent at Home Despot. Put two-inch foam board over the old siding, glued at the edges. Put 7/16 OSB nailer over that with long screws. Then my wife put up shingles, which she likes to do. The house was built of rough cut hemlock, fully four inches, which at R3.8/inch for cellulose is about R15 for the stud bays, and the foam board makes it R25. But it's the air sealing that does most of the work. It was still drafty until a couple years later when we put spray foam over the top several feet of cellar wall and over the joist bays. It's not hard. We did it ourselves with the canisters they sell at Home Depot. You have to suit up and wear a very good respirator. It's recommended that you use the kind with an air supply. I didn't, and survived, but I should have done. But you can get a contractor to do it for you and they have all the proper kit.