Always warm up with long bows in front of a mirror watching to see if you are pulling the bow at a 90 degree angle from frog to tip and back tip to frog. There are three sound points to consider:
1) pulling on the slight from middle between bridge and fingerboard towards the fingerboard on the lowest two strings, and slightly from middle towards the bridge on the upper two strings.
2) pulling literally on the bridge sul-ponticello on all four strings. You will get a fuzzy sound but a great boundary to use being next to the actual wood of the bridge.
3) pulling the bow near the fingerboard on all four strings using the visual of the fingerboard as a boundary. This is sul-tasto.
It is important here to have good mechanics with the wrist, elbow, and back muscles. Always use a mirror and not to rely visually just looking down on the bow on the string from where our head is on top of the instrument. Good luck.
Thanks,
JMF