After using a sewing machine that controls the stitch length and direction, to find your hands in charge of both of these things is a bit like when you're first in charge of steering a car - it feels very unnatural and achieving any degree of control feels a near impossibility. The fact is, however, that just as a learning to steer a car comes with practice and confidence, so does free machine embroidery. The great thing about making thread bowls is that what might be perceived as imperfect first stitching adds to the texture - as this photo shows.
The starting point is to take a piece of water soluble fabric that is double the size of a large embroidery hoop - at least 12" in diameter. I like to use 'Avalon' water soluble fabric which is easy washed away in cold water. Fold the fabric piece in half and lay on top of the hoop. Next scatter a few fibres, threads or ribbons in between the two layers - very little is needed, just something to add a bit of substance. Then trap the double layer of water soluble fabric and fibres within in the embroidery hoop - pulling as tight as you can as you tighten the screw on the side.
You also need to be able to drop the feed dogs on your sewing machine - the teeth on the needle plate that normally pull the fabric through as being stitched. If you don't know how to do this, refer to your instruction manual or have a look around your sewing machine for a button - it is sometimes well hidden on the back. If you find that that feeds don't drop, you may be able to purchase a 'darning plate' or use a substitute for covering the feeds up.see this post for advice on sewing machine needles.
There really are no hard and fast rules - wavy lines, big or small loops - just do what you feel most comfortable with. The harder you press on the presser foot and the slower you move your hands, the smaller the stitches will be. Light on the presser foot and fast on the hands creates big stitches. Experiment to see the different effects. You can also use pretty much any thread you like - you just need to match the needle to the thread.
Making thread bowls is a really fun and creative introduction to free machine embroidery and it is very motivating to make something with your first attempt that you are happy with. They are perfect for placing a small gift in or simply beautiful on their own. Do give making them a try and I would love to see photographs of finished creations.