All the fin and wing assembly pieces are squared up with a sanding block.
Study the picture below to see how the parts fit together.
There are three rectangular pieces, the longest one goes between the rear of the wing and the stabilizer. The two shorter rectangular pieces are used on the ramjet pods.
I chose to round the leading sides and trailing side edges.
Don't round any edges that will have any strips glued to them. All edges that are vertical in the above picture should be left square.
It's up to you, the edges could be left square.