How do I cut this large piece of fabric straight on both sides (on the selvedge sides)?

Fold it? I don't know. Also, my plan is to sew two 42" pieces together (for width) so I'll have a center seam down the back of the quilt.

Just not sure how to go about doing this and I don't want to mess up $60 worth of backing fabric!

Thanks,
Michelle

depends on whta size you are sewing. When I did my queen I just folded the fabric together lengthwise right sides together. On one side of selvage I would sew about an inch cut along fold, and cut off selvage. Dont know if this helps or makes sense

I've read that it is better to have two seams rather than one centre seam. You cut off both selvedges from one length, then half the other length, and sew the cut edges together so that you have one full width panel in the centre, and two half width lengths either side of that.

Leslely is correct.

Piece a Backing for Large Quilts
Pics help me to understand things, hope this helps you.

I sew mine together...sew it into a tube, then recut...you will have two seams that way you will not have a seam running down the middle.

What I do:

Trim yardage to twice the length of backing needed (plus a bit extra if you’re like me)
Trim the cut ends of yardage so they are squared off
Fold yardage in half on crosswise grain, right sides together
Match up selvage edge on one side and pin from cut edge to fold
Stitch down pinned edge to fold using a 1” seam allowance
Trim fold off square to seam just completed
Trim selvage off leaving a ½” seam allowance
Trim other side, before opening, so that total width from seam to edge is one half desired width

For two seams, as Lacelady suggested, pin and sew down both sides, trim the selvages and the fold as above so you have a tube, and then cut up the center of one layer.

The only real trick doing it this way, is making sure everything is squared up before you sew the edge(s) closed.

visit John Flinn's web site- he has a great free tutorial on piecing a back that saves fabric- and disperses the bulk so it lays nice and straight.
and it does depend on how the quilt will be quilted- are you hand quilting it? or, are you quilting it yourself on your domestic machine? or,
are you planning on having it long-arm quilted?
if you are planning on having it long-arm quilted it is best to have your seams horizontal- not vertical-
the quilt is loaded on rollers- when the seam is horizontal it will lay nice and straight along a roller- if the seam is vertical it builds up bulk (4-layers at a time) with every turn of the bar---which can really build up-and cause floppy sides.

Ditto for a pieced backing. Make a tube, trim the selvages and cut down the middle.

I prefer to use the extra wide backing (108" ) because I don't like seams on the back.

Check with your LAQ, she might have a preference.