if you are sewing with a silky fabric such as organza,batiste or polyester, the fabric might pucker with too short a stitch, so i'd try 8-9 stitches per inch, or experiment on scrap with the machine basting stitch. if the fabric is cotton i'd go a little tighter, maybe 10-12 stitches/inch. just watch your tension. if the fabric starts to pucker or draw up, try a longer stitch length or loosen the tension. i'd use a straight stitch either way. also use a polyester thread especially if you will be machine washing the curtains later. cotton thread might shrink some and also cause puckering. happy sewing. i hope your curtains turn out nice!
I agree with debbie, and I would add a wash away stabilizer if you still have problems.
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