The nature of star formation in high-z galaxies and the physical properties of the interstellar medium in which it is taking place are not very well understood as yet. Observations have shown that high-z star-forming galaxies often exhibit turbulent, clumpy, rotating discs, yet the limitations of current telescopes hinder the study of individual star-forming regions within these galaxies. The strong lensing phenomenon magnifies background galaxies located behind rich galaxy clusters, providing an excellent opportunity to study distant sources with enhanced resolution and flux. A notable case is the brightest lensed submm galaxy at z~2.8 behind the massive Bullet Cluster, which has an exceptional differential magnification up to 50 (SMMJ0658, z = 2.778). This poster presents the ALMA data reduction of SMMJ0658 (2015.1.01559.S) with CASA, including the CO(3-2) line moment maps, and the reconstructed lensed galaxy in the source plane using the software Lenstool, and its Python wrapper PyLenstool.