1) make sure that the 2 strips you are joining are the same width before you start. If you have a jelly roll, they may not all be the same!
2) Refer to the excellent photo in CarolynMT's post. Don't try to join the end, move in a half inch or so.
3) MARK the 45 degree line where you are going to sew.
4) Use the 1" marking lines on your cutting mat to make sure that when you lay the 2 strips together, they are exactly 90 degrees to each other.
5) PIN!
6) When you sew, the needle should enter exactly in the corner where the 2 strips meet and exit exactly in the opposite corner.