Pillow Help Please

I am trying to make Netherfield Park into a pillow, and right at the get-go I am in need of assistance. Should I wash the pillow fabric before I sew it to my stitched piece? I'm going to "frame" the pillow with fabric rather than using it just as backing. And any tips on cutting the fabric straight would be most appreciated. I pulled threads in the stitched piece to get a straight cutting line, but that doesn't seem to be possible with my fabric. Thank you to any more experienced soul who can point me in the right direction. I was not a seamstress in a previous life!