there are a couple good tutorials here on the board for doing prairie points- go to the tutorial section & search prairie points- then you can see exactly how to do them- i've never heard of cutting off your corners the way you have described. i baste the prairie points in place- stitch the binding over them- then when the binding is folded to the back the prairie points (stand up- fold out) and i hand stitch the binding to the back...
anyway- check out the tutorials - hopefully you will find help.