Hi
What are you trying to teach? The down on her side, or a roll over?
Dogs can lie down two ways, the Lion down, which is on their paws, the 'flat' down, on their side.
You can also teach a rollover, although you lure this and don't really need a reliable down to start with.
Do you clicker train?, if you do, this makes it easier, if you don't, then you will need something that motivates your dog, either a toy, or a treat - Make it a high value one.
Flat down. Lure your dog into the down, by getting the sit and using your treat to bring the dog slightly forward and down to the floor. Once the dog is lying down, have a look at the legs, is one leg slightly under the body, because you need to get the dog to lie with the leg that sticks out, out. Then very gently place your hand on the side of your dogs shoulder and keeping the treat under their nose, encourage them onto their side. Don't push or shove, (if the dog resists, then let them go where they are comfortable to start with and treat that. When they are happy with this, you can move it on a bit further.)
As the dog goes on their side, give the treat, keeping your other hand on the shoulder, giving it a rub, or rub the ear/tummy/whatever gets them relaxed. When you are happy the dog is relaxed. Stop praising and rubbing and give a 'release' command i.e. OK, That'll do...
Do this a few times and then you can start to introduce the command. Usually a different one from the 'lion down', I use 'flat'. Next stage is to get it so you can remove your hand fussing them for a few seconds, tell them flat and stop rubbing for literally five seconds, stroke calmly again and then release them.
Within a few days you will have your dog going down to a flat on just a command and staying for several seconds. You then progress this to you standing up and them staying there etc. etc.
P.S. If your dog at anytime resists, or gets worried, go back a stage to where your dog was comfortable and progress more slowly.
Clicking makes the whole thing much easier and you can allow the dog to offer the behaviour after the first few times and as they think it out for themselves they tend to remember it better.
Roll over is a bit different and hard to explain, but you can lure it from a lion down (you don't need to tell your dog down, as you want it all in one motion for the end result) taking the food/toy, in a circular motion over the head to the other side. They can be a bit 'dim' at first and just swap their head frontways to the other side, but keep going right over and try treating part way over and they soon get the idea. It is usually easier for dogs to move anticlockwise for some reason?!
But look at the legs in the down. to see which way is going to be easiest for the dog.
Sorry it sounds a bit double dutch, but I have just done it with my dog and that was the best I could come up with, always better to have a demo!
Kate