We can see our geometry is a bit messy at the joint so we need to clean it up. Zoom in to that region, select the extra Edges and hit Backspace to erase them without creating a hole in our object. Repeat this step on the other side if necessary.
Next do the same in Vertex sub-object mode. Check the geometry against useless vertices on both sides.
This is how your geometry should look like after you cleaned it.
We have just opened our tutorial directory! Feel free to submit tutorials hosted on your website or other online tutorials that you think are worth mentioning.