At the tip of each end, mark a line on the face of the tip so that there is a 1/2" section, basically this is where the paddle will taper down to, on each side make a mark at the same as a reference point for where you are going to draw the paddle line on the side, turn the paddle on it's side and then using a ruler draw a line from the end of the paddle handle line down to the 1/2" edge, it's like forming a V on the side of the paddle and this is will be your guide mark, do the same on both sides of the paddle on both ends. The picture below shows a cross section of what the above describes.
Now basically you plane the wood down to the line, shave everything off the top and bottom on both sides of the paddle, from the paddle handle line back to the top, most important part here is to make sure the lines are perfectly straight and even on both sides using the pencil lines as a guide, this will form the basis of the paddle, any difference will show up instantly. I started using a hand planer but was new with it and was difficult going, went with a power planer and made quick work of it, double check on both sides that you are planing evenly no matter which method you use, takes time but go easy and with the grain and it will shape up nicely.
Having since learned to use the hand planer, because you get more control I would use that now for sure, also double check by sliding a ruler across the paddle survace as the ruler will show gaps, show be equal across the paddle with now bowing or light showing through. Smoother you make this the better. I have mine a quick sand once completed.
Now have a paddle blank and the fun begins...