Preparation techniques could fill a book, so let’s just focus on the high points for now. First you need to sand all surfaces evenly, working your way up through sanding grits from course to fine, skipping every other grit: 60, 100, 150, 220. The idea is to use 60 grit to eliminate tool marks and flatten large surfaces, then use each successive grit to eliminate the scratches left by the previous grit.
On flat surfaces back your sandpaper with a sanding block. Use a shop light and sight along the wood. When you have an even sheen with no scratches left from the previous grit you can move on to the next grit. When sanding by hand, do not use a lot of pressure. Just the weight of your hand and even strokes should do the job.
If by chance, you decide to use a power sander, I highly recommend a random-orbit sander because unlike a belt or vibrating sander, it does not leave a noticeable scratch pattern. The key to remember is, you do not need to apply pressure. The weight of the tool is sufficient to finish the job.