We present the stitching interferometry system of ALBA. It has been designed for measuring the surface of long X-ray optics (up to 1.5 meters) with sub-nanometer accuracy, thanks to effectively removing the systematic errors introduced by the flat reference. We discuss the main features and measurement routines of our setup, including the aspects related to error removal. The instrument is based on a Fizeau interferometer, Zygo Verifire HD, with a 100 mm aperture, which is used to take different sub-aperture measurements. The interferometer is mounted on top of an in-house built scanning and positioning stage with four degrees of freedom: horizontal displacement, vertical displacement, yaw rotation, and roll rotation. These four degrees of freedom are essential for obtaining sub-nanometer accuracies. Horizontal and vertical displacements are needed to remove ambiguities in the surface reconstruction, and yaw and roll rotations are required to always align the interferometer to each sub-aperture to minimize retracing errors. The relative orientation between the optical bench and the interferometer platform, which comprises all three orientation angles, is measured by an external autocollimator and two inclinometers. Tracking the interferometer trajectory allows us to remove the guidance errors and solve curvature and twist ambiguities. All the stitching acquisition process is automatized. We finally show the first commissioning results and we discuss the factors that limit the current accuracy of our system. In the first results, we have reconstructed the reference flat with a repeatability of 50 picometers rms.