First, Regulating the Length of Stitch. See the lever in the round dial in the above picture? It's the Stitch Regulator. The numbers indicate how many stitches per inch (from 6 to 30 on this machine). The line above the numbers is the neutral position. The screw on the left in the curve is the thumb screw.
To change the stitch, loosen the thumb screw and move it to the bottom of the slot. Move the stitch lever until its top is in line with the number of stitches you want. Next move the thumb screw up until the plate inside touches the lever and then tighten the screw. That's it.
Having the thumb screw tightened in place is what makes it possible to go in reverse while keeping the stitch length the same. To reverse, simply push the lever to the top of the dial. If you don't care about reversing just leave the thumb screw at the bottom and move the lever to whatever number you want.
On later model Singers the stitch regulator looks like the one in the next photo. Twist it to loosen or tighten. Again, pushing it to the top reverses the stitch.
On much earlier models the stitch regulator was a turning screw with no markings as in the next photo. Turn it to the right to lengthen the stitch and left to shorten. You can't reverse on this machine. Some machines (the 15-30 and 115 models) had a small screw in a slot. Loosen the screw and move it down to lengthen the stitch and up to shorten; when you have it where you want it tighten the screw.
Here is a chart with recommended stitch length for different weights of fabric along with topstitching lengths. It's supposed to enlarge when you click on it. I keep my stitches at about 10-12 for general sewing and quilt piecing.
You can change the force of pressure the presser foot puts on fabric while sewing. The fabric is supposed to feed evenly through--you know that. If the pressure is too light there might be an irregular stitch length and irregular joining of seam edges. If it's too heavy the machine can run "heavy" and the mark of the feed might show on the fabric. Again the seam edges might be unevenly joined. There is a thumb screw on top of the Presser Bar for adjusting. Turn it to the right to increase pressure and to the left to decrease.
The type of fabric determines the amount of pressure needed. My Singer book says, "The heavier the material, the heavier the pressure; the lighter the material, the lighter the pressure." There is an exception: for pile fabrics, such as velvet, velveteen, and corduroy, use a lighter pressure than you would use for the same thickness of flat woven fabric. Honestly, I don't change my Presser Bar. I have it set for quilting cotton type fabrics. I did sew on some velveteen awhile back, but I used a walking foot for it--no adjustment necessary for that.
There is a test for pressure adjustment. Stitch lengthwise on a doubled scrap of fabric without thread. My book says, "When all seam edges are handled evenly the correct pressure adjustment has been reached." That seemed a little vague to me, so I popped over to my machine to try it out. I found when the pressure is too light the fabric kind of slides around unevenly under the foot when I'm only lightly touching it--this would make the stitches irregular. Having the pressure too heavy is difficult to describe. The machine did seem to run a bit "heavy" and the fabric didn't feed right, almost sluggish--it's almost as if you can sense it's too heavy. I have it set right again and there seems some leeway to adjusting it. I think if your presser bar is too far off you will notice it. Don't worry about it otherwise.
What next? A look at feet.