Bowing

Keep the bow straight - no curving round as the bow goes up and down.

Keep the wrist loose - imagine your hand is like a string on the end of the bow and let it bend with the motion of the bow.

Do not press down too hard with the bow - let the bow glide over the string and concentrate on tone, with no unnecessary abrupt start and stop points. Emphasis can come later.

Fingering/posture

Keep the fiddle up and the left elbow tucked in so that the hand is curved round and down on the fingerboard.

Keep fingers vertical.

Do not raise fingers off the fingerboard unnecessarily - keep them down where possible - which helps with intonation and speed.

Play scales in different keys - constantly.

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Just some tips from my own learning.