What about muslin?
 

I want to do a "string diamond" pattern. The instructions say to use muslin as a foundation. I've never worked with muslin. When I started shopping on line, wow they really range in price. Advise?

how about this? http://www.quiltfabriccloseouts.com/...es=0&lastmenu=

I use already used dryer sheets for the foundation & so do many others on the QB. They're free & work great. For a lap quilt size, it takes about 94-96 of them. I save the sheets & now have a big "supply" for the future.

I'm one of those that would never use dryer sheets because of the inherent perfumes, chemicals, etc. doesn't matter if they've been used, I'm not comfortable handing that issue off to anyone in a finished quilt. Not with the allergies and sensitivities, asthma etc. that are common today. You can get fairly inexpensive muslin anywhere. It doesn't have to be high grade because it's being used as a foundation base.

Muslin like any other quilting fabric comes in a wide range of thread counts. The higher the thread count, the better quality the muslin. When I have used muslin as a foundation fabric, I have used a mid-range thread count. The cheaper muslin tends to be flimsy and cheap. Just my opinion.

i just got some for 1.29 a yard and it was good enough for lining the drapes was making.

If its being used for foundation , I would not be fussy . But Do preshrink, the lower quality will shrink alot more than the more pricey.
Really any fabric can be used for a foundation( as long as its washable) . Its a good way to use up some of those "what was I thinking" fabrics.
Some use regular copy paper , using a small stitch like you are paper piecing. Then remove the paper once its sewn into the block shape. I know some thrifty people who use newspaper ... I am not a fan as my newspaper the print rubs off too easliy .

I use liquid fabric softener. I really don't care for the sheets. But I do have "what was I thinking" fabric. Should I use that? It is cotton.

Only if the fabric is light with little patterning. If you have pieces of your quilt top sewed down to it, the fabric could show through any lighter quilt top fabric. Does that make sense? That's why most folks use muslin or white fabric as the base. Now, if you're using dark fabrics only on top it wouldn't matter as much. Though it would be safest to pick out your top fabrics and place them over your 'what was I thinking' fabric to see if it shows through.

Thank you so much for the advise. It makes sense. Yes it is white, solid. Too thin for me to quilt with. Almost like a shirt fabric. This is my first string quilt. THE INSTRUCTIONS SIMPLE.