Here are the methods I use
Several layers of material
My pictures always consist of several layers of material. Lowest are two layers of cotton - old sheets or the like - colour unimportant, preferably ironed and cut roughly to the right size.
For abstract pictures the pieces of material which form the pattern are fixed directly onto the sheet with pins, with the edges slightly overlapping. It's important to complete all the areas for the entire picture in this way before sewing - otherwise it's difficult to achieve composure/harmony. The pieces are then zigzagged on, with very close stiches and a neutral thread.
With 'figurative' pictures the method is the same except that a piece of material in the background colour is first fixed to the sheet. The complete picture is set up with pins before sewing. In practice I often wait many days before sewing since it's very difficult to undo if something is wrong. Of course the first idea is not always the best one and usually bits have to be moved or discarded and something new added. With these pictures the thread is often changed to a new colour, but only the over-thread. If the machine is correctly adjusted, i.e. with hardly any tension on the over-thread, the under-thread is unimportant.
Drawing with the thread
Sometimes the sewing thread is used to draw with. Here it's just a question of zigzagging with varying width. This can be seen for example with hair, with certain facial features and in the letters of a text.
Pins can be a problem with big pictures. They fall out easily when using the sewing machine and I have therefore got into the habit of of stopping frequently to check that all pins are still in place so that the picture doesn't begin to disintegrate. I sometimes use a glue-stick instead of pins for very small bits of material.
Lastly the picture is mounted with a frame of material which usually covers the entire back and some of the edge in front. Cloth for the frame is the only new material I buy, although of course there are bits of new fabric in the pictures as I'm often presented with remnants. Otherwise most of the pictures consist of secondhand, used material.
The pictures hang from a round pole inserted in a hem which forms the upper edge of the frame.
Some pictures are failures, but as a rule there are bits which are useful. So instead of discarding the whole picture I cut the good pieces out and use them again.