Bow's 'Sweet Spot' 
This is the 'centre of mass'  point of the bow,  it's balanced 'self weight' point before adding the weight of the right arm, hands and fingers or placing on the string. This is the sweet spot  where the bow  moves naturally and effortlessly in all three planes  :- tip up, tip down; tip  left, tip  right, tip forward, tip back. Grab the bow with the left hand to bring the 'sweet spot' to mind before placing any bow fingers.  BALANCE IS PRIOTY No1 in all bowing  and the 'sweet spot' is a key node like the D stop is with the left hand.

Natural  Bow Hold
From making relaxed circles with A COMPLETELY relaxed arm, simply throw the hand up to the frog and just let it land in its natural position (hold the bow with the left hand at the sweet spot). It should feel relaxed with no grabbing distortion of the hand - just your natural relaxed hold. Do it several times to find the best "naturally you" position - everyone is different. See where you naturally go as the hand comes up to the bow nothing more. It is a delicate hold like holding a pencil. Every hand is different and so theres no 'one size fits all'  image of what it should look like and this is just the broad brush relaxed starting block  which will be finessed but the key thing is to be relaxed..

The Lightest touch
The idea of one thing on one side and one thing on the other is key. The 2nd finger and the thumb are these two opposites that enable the lightest of holds on the bow with all the other fingers away from even being in contact with the bow. Have a good couple of inches separating the thumb and the 2nd finger building on the form of the hand in the natural bow hold.  This is the ultimate 'light touch' position  where the bow can be held in balance  similar to holding it in the Sweet Spot with the left hand. i.e. theres no pressure applied - just the weight of the bow is supported on the thumb and 2nd finger. Work this up from the Natural Bow Hold exercise.

Up with 2nd, down with 1st
The 2nd  is the key finger to place as it is the main load bearing support of the bow and the weight is transferred to the back part of the thumb. On a German bow its positioned located  in front of the frog on the stick itself and on the next facet up from the underside facet.  The first finger then goes on the first facet down from the top facet forming a two finger clasp of the bow on one side with the thumb on the other  (NB the 2nd finger is below the vertical side facet - supporting the bow , and the 1st finger is above the vertical facet ready to apply pressure ).  3:30

● this tutorial is useful for all who struggle with bow change control: from beginners until advanced 
● applicable also for violin, viola and cello
 ● very shortly before bow change, you will have to slow down the arm and even stop it for a moment before moving it into the opposite direction. At that moment, start to use the fingers & the wrist to keep bow stroking in the same speed, until the arm is ready to stroke in the opposite direction 
● the loudness and tone character should not vary at all during the bow changes and it should also stay identic during the whole bow stroke 
● it is utterly important to count very strictly every bow stroke and keep the bow speed absolutely identic whole the time 
● play with fair amount of weight, full contact, lead the bow slow and close to the bridge, medium until loud. Tone quality should be as good as possible 
● keep always full contact of the bow hair with the string, no "airy" sound at all, keep the bow in right-angle against the strings (90°)
● don't play just with corner of the bow hair (like majority of people does), use full width of the bow hair on the string 
● the angle of the bow around its own axis should not vary at all during the bow changes and during the playing, so don't rotate it, especially not during the bow change 
● the sound during the bow change should remain as even as possible: no accents during the change and no gaps in the sound or temporarily softer loudness, try to figure out how to sound well without reducing the weight during the bow change 
● the result of this practicing should be noticeable already after only few days. It is not only better bow change that he want to achieve with this, but also a much better bow and sound control, good luck 🤗 people who practice this hard should post their observations in the comments after few days, please

TUTORIAL: BOW 02 ● two crucial elements here: crossing over the strings, as well as the (very) beginnings of the tones / of a bow stroke 
● the result and goal is reaching of a focused, very well-defined tone - but without scratching noises at the beginning 
● applicable also for violin, viola & cello
At the very beginning of each bow stroke, the bow should be positioned (laid) on a string. Start stroking the bow very light-weighted, then right after use full amount of the weight (for the respective dynamic). Thus during first 1-2 mm the weight is increasing, like in a crescendo, but it happens so quickly that the listener should not recognize it. That way, one can control the sound quality very effective way. In opposite, if the bow lays on the string with its full weight and the stroke begins afterwards, a scratchy noise might occur. The danger is particularly with sticky rosins "for double bass" (the brands claim that bulky thing that way, at least) .
The very same thing when crossing the strings, just: ● a peaceful, comparably slow changing of the arm angle is needed by crossing the strings. Practise the timing of the arm angle adjustment the way that you get the next tone to sound exactly when it should (tempo wise), but mind that the arm angle change should begin (timing wise) earlier to keep the move as peaceful as possible ● While the bow begins to touch the next string, it will still actively stroke the previous string. That is totally normal, although the listeners should not hear it that way. The artistry here is to produce a very beautiful legato sound. Don't interrupt the bow stroke, keep it in constant speed and try to keep the weight as stable as possible during the strings crossing. By doing it the correct way, you will reach within the bow stroke the same effect I described for the single tones.

TUTORIAL: BOW 03 - DOTTED RHYTHM ● applicable for all strings ● so, now all you people who have been practising in the past weeks my previously uploaded tutorials can test how accurate and how patient have you been working. This is a very simple bowing vatiant, often used in the orchestral literature. Yet, this one is usually nearly unplayable for people who struggle with the bow change.
Between the two fast tones is the bow change contains exactly as I described in my first tutorial video BOW - 01 ● this video displays three stages of the practicing: the very slow one (correct way to start with), the middle and the final tempo stage ● it is totally individual how much time is needed to get further with the next stage. Tenths of hours will be necessary usually, some students can make it in a few days and another will need few weeks. Taking care of the highest precision during the bow change is a must. Increasing tempo stepwise is much better ally than an impatience ● dotted rhythm should be performed at all stages: and it should never be tripletized (rather overdotted) ● the faster the tempo, the amount of the bow gets smaller. One can see in this video also how to act when a string change happens between two fast notes, that was the content of the tutorial BOW - 02
As an example I have played here one passage from the final movement of Schubert's Symphony in C major. My apologizes for people with absolute hearing, I also hate "solo tuning" strings. This video was also recorded during a "real" lesson and I demonstrate to my students on the instruments they use.