Afghan border

I have knitted a 12 block afghan and joined the blocks. The instructions state to add upper, lower and side borders.
I have done the upper and am finishing the lower before I start the sides but cannot figure out how the borders will meet at four corners.
I have not knitted for about forty years and then only knew basics. I have tackled this intermediate project and am satisfied that it is OK even with
varied knitting and errors.
I hope you can help.
The afghan is from Better Homes and Gardens Kit Lion Brand Multi Square Throw- October or November 2005
Thank Youn
Peggy Taylor

How did you work the ends? If they’re in line with the sides, then have your side borders go all the way from one end to the other, including being a border of the end borders. Does that sound understandable?

Thank You.
I will try to complete in this manner but will let you know if this does not work.
Peg T.