These are good suggestions.
Many of us who sew with non-computerized machines sometimes forget that the 'thread take up lever' -- on the front of the machine that lifts the thread before it goes into the needle -- needs to be in the "all the way up" position before starting to sew. When it isn't, it tends to grab the bobbin thread and that cause a bunch or knot underneath the fabric. To make sure the lever is up, you can hand turn the wheel on the end of your machine -- ALWAYS toward you -- before you begin to sew.
Jan in VA