Hi Sheila,
When I quilt diagonal lines on my quilts, I first put a strip of painter's tape (the blue stuff), from one end, like top left, to the other, bottom right. Once I've quilted that line, I take up the painter's tape and reposition it along the opposite corner to corner diagonal line of the quilt. This makes an 'X' across the quilt.
Once I've done that I just keep repositioning the tape from these center lines out the way out to the outer corners. Repeat for the opposite side. I learned this technique from Harriet Hargrave's "Heirloom Machine Quilting" book, a must have, I think for any aspiring quilter.
NOTE: For a rectangular quilt, you can make two 'X's over the quilt top. Hope this makes sense.
Have fun and keep us posted.
Happy sewing,
Karla in CO