Sewing seams is a vital part of making curtains. You'll need to practice making neat joins when you sew fabric widths together. Once you have drapes with more than one width in them it's important to master this part of drape making.
If using a sewing machine is new to you, then get lots of unwanted fabric and practice sewing seams. Cut the fabric into small strips, say 6" (15cms) wide. Then just practice sewing them together. Use decent lengths as well, the longer the better. That way you'll be able to see how straight you can sew over longer lengths. Remember, if your drapes are 8'0" (2.50m) long, you won't want to see unsightly, wavy joins!
The tension in the top and bottom spools should be balanced. If they're not, you'll get all sorts of weird stitches which will pull and pucker your work. Spools have small screws on their sides which you can use to adjust the tension.
Remember to make sure that the take-up lever is fully up before you remove your work. If you don't, the threads will still be in tension, and you might even break the needle.
When removing the fabrics, always take them out towards the back of the machine.
Strain on the needle
There should never be any horizontal stress on the needle when you're sewing seams. So always support large pieces of fabric. Don't let the needle be put under any strain. If necessary place extra tables next to the machine to support the fabrics.
Mastering the feed mechanism
Assuming you're using a normal domestic machine and not a needle feed one, the fabric in contact with the feed mechanism (the teeth which are level with the footplate) may be pulled through faster than the top fabric. This can produce a puckering or gathered effect - not what you want!
To prevent this, hold onto the bottom fabric and guide it so both pieces go through at the same rate. This just takes practice. You might need to work out the best technique for your machine.
Get a 'feel' for your machine
Leading on from the last point, work with the machine. Don't try to force fabrics through. Try to get a 'feel' for the machine, and work with it.
Different weight fabrics
When you come to sew a lighter fabric with a heavier one, place the lighter fabric on the bottom next to the feed teeth. As before, if there is puckering, pull gently on the lighter fabric.
Constant seam allowance
When you machine sew, sometimes you might want to make sure your seam allowance is a fixed amount. Stick masking tape onto the footplate at the required distance from the needle. By using this as a guide you can keep the seam allowance constant.
You'll find a lot more information about sewing and sewing machines at Sewing Basics. (This link opens in a new window.)
Don't be dazzled by the technology of modern sewing machines. It takes skill to get perfect joins, and that takes practice.
Is your machine old? I've seen skilled people get perfect results from 60 year old machines.
Get used to your machine. Get to know its little ways, and you'll soon be achieving perfect joins as you machine sew.