let's see... so you have a seam down either side and then an open part... take the middle of each side of the open parts and pull in opposite directions, this means the two seams are now lined up touching, sew across there... i think that probably sounds more confusing, rather than less confusing, but maybe it helps?
here's some pics to help (maybe)
so you should have something that looks like this (the white part are the seams)
then just hold where the red dot is (on either side) and pull them away from each other and then you have a little line
where the dot is will be the start and end of the line
this is why i don't write tutorials myself