The specified preload is ground into the pair of bearings. When you clamp the inner races together in a back to back (DB) pair, they are preloaded.
Clamping the outer races to a fixed shoulder in a housing prevents axial motion of the ball screw.
No shimming is required.
ADD: The SKF bearing numbers listed above do not indicate preload or accuracy class. 7204 BECBP indicates a 7200 series 20mm bore, 40 deg. contact angle, paired bearings with a glass fiber cage. Missing are grade such as P4 and preload letter such as A = light preload; B = preload greater than A; C = preload greater than B; CA = small axial internal clearance. This info is from an SKF General Catalog.