Edit: Don't use water on the frame prior. Follow the instructions. Water and varnish don't go well together. Your challenge will be placing the decals accurately as once they touch the frame there is no moving them. I think I'd be tempted to carefully slice the down tube decal into two sections to make it easier to install, but this could certainly backfire on you, showing a seam down the middle.
Instead, I'd carefully lay out the centerline of the tube with pin striping tape, just above and below the decal's final location, and then get that big decal centered well and touch the frame first on the center line. Burnish down lightly each side as you lay it down. Sounds like four hands might be handy for this part of the operation.