You have probably progressed beyond needing this advice, but if not, here it is:
Get your plans off the internet, from books or copy an existing paddle that you borrow.
There are a number of ways to create the ridge. You can cut the tapers into the blank, glue on a narrow strip for the ridge and then fair the "glued on ridge" into the blank.
Another good method for those skilled with a router is to make a paddle core the thickness of the loom, form the ridge on this paddle core with router and then glue on paddle sides for the blades. Finish the paddle with hand tools. I was taught to use sitka for the paddle core and have made sides from red cedar, yellow cedar and redwood. With the strong sitka core, the blades are replaceable if damaged badly.
I've mostly band sawed out the blank and formed the ridge with a scorp. Others have formed the ridge with radius bottom and/or curved bottom small block planes( sold by Varitas or Lie Nielsen). When teaching a friend to make a paddle while also making one for myself, we took turns with the radius plane and the scorp, both preferring the scorp. This scorp was made by North Bay Forge and has a tighter radius than the more common chair making scorps and one handle not two. It does have a wider radius then the small carving scorps now being sold in woodworking stores.
I've read of people forming the ridge with a gouge chisel and also with the Stanley Sure-form, a rasp like tool. I have not tried either of the above.
Once the ridge is formed by whatever method, the paddle is shaped by spokeshaves, block planes, rasp and sandpaper. ( I totally avoid power sanders as it's very easy to become allergic to red cedar dust and I have). If you choose to disregard this advice, wear a good mask, throw clothing worn into the wash after each day, throw away the mask used that day, take a shower and last, swab out your nose with a wet q-tip to get the dust that got under the mask.