So hopefully at this point you have now joined your pieces together for your soundboard and/or the back of your guitar. Also hopefully you did not leave the pieces overly thick but at least thick enough to make sure you could sand out any unevenness in the joint you made. Most likely your plans will have recommendations for thickness of your soundboard or back of your guitar. I’ve seen anywhere from 1.6mm to 2.5mm.
If you are loaded and a ton of money you wish to just invest in wood working tools, then now would be a great time to go by a really nice thickness sander. Here is a nice one, it costs about $1,229.99
Of course if you are like me you either don’t have the money or you have other better things that money could be spent on. So let me tell you what I came up with an alternate option that is a lot better than going at it with a sanding block. Specifically I went and bought myself an inline sander at Harbor Freight along with some wood sanding belts that I cut to fit on the inline sander. Typically and incline sander is meant for use for body work on a car, but it serves its purpose for thickness sanding wood as well. Now obviously if you don’t already have a good air compressor you will need to get a decent air compressor to run it as well. But in the end you still will spend a lot less than a thickness sander and the air compressor can obviously be reused for many things.
First you will need a way to hold your piece in place. I opted to lay it on my wood work bench and drill holes and stuck wooden dowels around it to hold it from moving around as I sanded it. Now you can use your inline sander to go back and forth over your board to thickness sand it. Because it has such a long sanding pad it is easier to keep everything even across the board. Another option that I have seen others do when hand sanding and trying to achieve a uniform thickness is to use a drill press with a stop to put holes to the exact depth they want all over the boards.