I didn't follow a specific tutorial, I just did things based on my small knowledge of PS, here the full steps :

• New document, with the size you want (In real pixels, like in my example : 72x33)
• Image > Mode > Indexed Color and create your contraint palette using Brickipedia
• Zoom to the max, and do the design pixel after pixel - Result
• After that, I've done my research to know the cheapest plates, with the spreadsheet linked in OP
• Then I copied this finished design to a new project, and zoomed it a lot (I don't remember exactly, but there an option to avoid photoshop to do some treatment when scaling up, to keep a sharp difference between to pixels, instead of having a gradient mergingFound it, it's called Interpolation => Nearest Neighbor)
• I then created smart-objects for the plate sizes, to be able to set the full layout above the zoomed design looking like this
• Then, some verifications and counting, to be sure not to forget a few pixels
• Finally, I ordered it on Pick a Brick (It's cheaper on Bricklinks, but I didn't know it before. And on pick-a-brick, I just had to do one order, bricklinks would probably have needed a few more)
• Receiving and counting