I have a very long in depth answer to your question for you, but my simple answer is when I ask a horse to lope I bring my "lead leg" so outside leg back slightly so as to ask the horse to step out with the back leg and hip first, I close my outside spur or heel, I prefer spurs just as an aid in getting the right buttons, I close my inside calf at the same time and hold, so as the front end will stay straight as I ask. I usually pick up with my inside hand slightly so as to lift the inside shoulder and soften or slightly bend my horse to the inside. I then push and hold with my outside leg and think of a count of 3 then release for a second and reapply my leg for the count of 3. So a push release and push release. You want the horse to sit back on their hip, lift through the shoulder and step out strong from behind. That is what your leg is telling them.
I hope that helps.
A "made" horse has had all the buttons and training put on them as well as been taught how to "go around." A green horse usually for the most basic reason of strength can not lope right away. Going collected, balanced and slower takes a lot more muscle and effort from a horse than going faster or forward. Before you ask a green horse to even try and collect into a lope you need to make sure you have built up enough muscle and balance so that the horse is able to try what you ask them. The most frustrating thing for a young or green horse is when you ask them to do something that they physically can not do yet. That is how you get some behavior issues and work ethic issues as you go as well. So here is my rule of thumb;
A horse must be able to do a forward lope or canter both directions, picking up the correct leads, holding themselves up, such as no leaning, a nice balanced feel as well as a consistent rhythm before I will ask one to begin to collect and slow to a lope. You also have to remember that you don't want to sacrifice movement for speed. So not all horses can lope at the same speed, and stay at a "true lope" and good legged.
With all that first, then I have a very long and structured teaching system I take a baby or green horse into to teach them all the cues they need and how to break apart the body into pieces to then have the ability and the horse to have the knowledge to understand what I am asking when I do ask for a lope, so a baby or green horse unless taught will not understand any cues you use. Also, keep in mind it takes time for a green or young horse to give you the "finished" lope and you have to go slow and build a young horses confidence. A lot of trainers and people want it done to fast and at the cost of the horses expense.
So remember to keep asking and stay calm and quiet with a green horse and it will come along faster than you think.