On the first day of school, I taught students to hold their pencils like a bow...and I start that by having them bend their thumbs and bring the pencil to the top of their bent thumbs. So, students did have that experience before they had real bows in their hands.
(Sorry about the fuzzy pictures...my cell phone wasn't taking great pictures.)
First, you can stick a smiley face sticker or animal sticker on students' thumbs. This will bring their attention to their thumbs so they can check to make sure they stay bent. I have students make the 'fox' shape with their hands...making sure the sticker on their thumbs are upright.
Next, I have students hide their stickers with the middle 2 fingers. They should be able to peek-a-boo by showing their neighbor the sticker, then covering it by bending their middle fingers over the sticker. I like this step because one problem some students have is not bending their fingers enough over the stick on the bow.
Before students try to hold their bows, I draw the shape of the frog on the white board and explain exactly where they are going to be placing their bent thumbs...half on the frog bump, half on the stick. I then instruct students to hold the bow in front of them with their LEFT hand holding up the stick. Students then place their thumb first. They check to see if the sticker is upright and they should be able to see the sticker through the bow...like a window.
Students then hang the middle two fingers over the sticker, somewhat hiding the sticker. (They can play peek-a-boo again with their stand partners) I tell students to pretend they have one invisible finger between the index finger and middle finger and they must save that finger a place to 'sit.' This helps students create that little bit of space between the index and middle fingers. Violin and viola students then put their pinkies on top....close to the other fingers. (If they try to place their pinkies far away from the other fingers, the pinky will not stay bent.) Cello and bass students let their pinky fingers come down on the stick.