Lope, lope and lope some more. Don't worry about how rough the transition is until she get VERY comfortable loping. Just sit the trot, use legs and 'cluck' for the lope. She will soon learn that when you sit the trot and cluck that she is supposed to lope.
I try to lope very big circles in an open field. I NEVER do figure 8s but will make two or three circles to the lsft, come across my circle, drop to a trot for a couple of strides and lope off the opposite direction. Then, I do two or three circles that directiion, cut across the circle and change directions again.
This give me the opportunity to teach several vital things.
1) They learn lope departures.
2) The two or three circles give me the opportunity to improve their willingness to 'stay between my legs'. This means that I am working at 'bumping' them to the outside of the circle on the side that they want to cut in on and I am bumping them in on the side they want to drift out on. Within 3 or 4 sessions, they should be guiding 100X better.
3) Cutting across the circles and doing simple changes gives me the opportunity to begin teaching them to pick up a particular lead.
Now, let me explain what NOT to do!
When you are on the side of a circle where the horse wants to 'drift out' -- usually on the side of the circle nearest to the barn -- ONLY PULL ON THE INSIDE REIN ENOUGH TO GET THE HORSE'S HEAD POINTED THE DIRECTION OF THE CIRCLE . Then quit pulling. Use more outside leg or tap the horse on the shoulder with a short crop or bat to make the horse 'follow his nose'.
If you keep pulling harder to get the horse to go where you want. You will only teach the horse to 'over-bend' or 'rubber-neck'. If you pull harder instead of 'pushing' on the on the outside of the horse, the horse will never learn to 'follow its nose' and 'stay between the reins' and 'stay between your legs'.
These are 'biggies' and loping big circles out in the open and changing leads (simple changes) and directions teaches much more than just 'how to lope'.