Hand planing the parts for the base is a great intro into getting components ready for handwork. Here are a few hints:
Use the outside of the rails as your true faces since they will show and will be partially (or fully) smoothed from the prep process. The top of the rail should be the true edge. You will need to plane to width as well but this dimension can have an error or two in it and no one will know but you. I often leave a scrubbed surface on the interior rails…flattish is good enough, and it leaves and interesting texture to find when someone explores the piece in more detail in the future. The rails don’t have to all be the same thickness either, as long as the are close to 3/4 inch you can make your joinery come out just fine.
The true face and edge on the legs should be the inside edges since they will be accepting the joinery. Try to pick the uglier parts of board for these faces putting the pretty parts of the board on the outside. Don’t forget to leave the legs longer than your finished dimension. Make the cuts on the bottom and top as square as possible though since this will help with layout. You will need to surface all the legs. Take your time to make them as straight and square and even with as can be. True the face and edges of your legs first then go over the pieces to see if you can find the narrowest dimension of your square legs. This will allow you to keep your marking gauge on one setting and serve to simplify this chore.
Depending on how much work you need to do you may want to smooth your work after joinery or before. The heavier the material removal is going to be the more I lean towards before. As stated before select good stock and your smoothing stage should be pretty minimal and can be left till near the end of the project (which is nice since you are bound to dent or ding your project at least once between stock prep and assembly).
Planes don’t always do all the work for you when removing warp. Think about the material you need to remove and selectively focus on it. This is especially true on the convex side of the warp since the plane tends to just follow the crest. Planes really excel at correcting concave surfaces though so that’s the good news, just plane until you start getting full length shavings and you are home free. On the convex side take aim at the highest spots and expand them until your board becomes flat.
EDIT: I forgot to mention in the text that when you plane across the grain you are very likely to cause tearout on the exit edge of your cut. I deal with this in three ways, I chamfer that edge to give better support to the cut and to finish the cut before the edge. I then usually joint that exit edge as the true face to remove that chamfer and any light tearout. I use the opposite edge as my exit side when planing to thickness and tend to remove the section for my width last in those cases, this removes your last chamfer and any odd tearout that may have occurred.
Twist is it’s own beast to remove. Plane across the high points and then finish with full length shavings. Fortunately I have a really good example for the class. I remove it at the end of this video.
Keep your tools sharp and start making some serious shavings.
-- Make furniture that lasts as long as the tree - Ryan