Personally I have never been able to transfer the under the leg to an actual behavior because getting down and leg bent at knee etc becomes part of the cue for the dog but what works great for me but will never work in a class setting to teach is a technique called capturing. IT works well with dogs that understand clickers but if you are prepared with treats you do not need a clicker.
Because at it basics "down" is something that you need not teach the dog they already know how to do to, what you need to do is teach them to do it on cue.
step one is increase the likelihood the dog will lie down by rewarding it. When you now the dog is tired or other times you know the dog is perparing to lie down such as getting up on the bed etc. You reward the dog right as the go down. Lure then back up and wait for them to lie down again reward. Over time They should start doing this pretty quickly. Once that is happening you can start to add a cue just when the dog is about to lie down say down over time the down becomes the cue to lie down. It is a no muss no fuss method you just need to wait for the appropriate oppurtunity to train.
see How You Get Behavior Really Does Matter
How to Use Your Clicker to Capture Behaviors
HOW TO CAPTURE BEHAVIOR
How to Add the Cue