Snatches are a whole body motion that can roughly be described as 3/4 swing and 1/4 press.

Start by doing 1 arm swings brought to about shoulder height. See how the KB almost "floats" at the very top of the swing just before descending ?
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Now, swing to head height and instead of letting the KB reach its apex and fall again, pull backward and up as the KB passses chest height. This extra momentum will carry the KB to the overhead lock position.

A key tip is that as the KB starts to flip over the handle, push your hand up as if putting on a glove. This rotates the handle around the KB center and you punch up with your forearm to meet the KB before it flips over and down. This takes some time and practice to perfect.

Start with a light KB for 10 reps left arm and then right arm; Then repeat 3 times (decreasing reps per set). Snatches are deceptively tiring so build up slowly- don't push to failure when you start !!