The inverted triangle is really easy to do , the question becomes do you know how to divide up a pattern? First you would make the master, which is what the finished bag will look like. In other words it should be like your sketch. Then where your triangle is, you cut that out then you have the top and the bottom portions of the pattern, add seam allowance to where the two pieces meet and sew them together.
Think like a bag, anytime you want to divide a pattern, make the master then cut it up, add seam allowance.