Here are the dimensions of the cabinet.  I have supplied a side diagram with the major dimensions to make your life easier.  From the front side, all interior widths are approx. 20".  I say approx, because mine varied from 20" on most widths to about 19-7/8".  This was due to my poor measuring skills. 

I suggest having Lowes make four cuts, 20 inches apart.  This will give you 4 20"x48" boards and a strip about 16" wide.  By having them make these four cuts, you will be saving a fair amount of time and getting very clean cuts.  Have them go 20" from one side, cut, go 20 more inches from that same side, cut, 20", cut, and go over one final 20" section and cut.  This is all done while the board is lying on its long side.  This would have made my life soooo much easier...

Throughout this document, I have provided many pictures of the construction process.  Wherever possible, I also include measurements.  Hopefully, between the pics and the figures, you should be able to create this cabinet with less fuss than I did.  Your measurements will invariably vary from mine on some parts.  I apologize for any confusing explanations.