There are different styles of longlining for different purposes. Often, with young horses, I find letting the longlines come directly back to my hands without going through a roller or anything is best - because I have a very 'open' inside rein which works well to keep the horse relaxed and loose and gives a great emergency stop if needed. But, the downsides are you need to be capable at handling the spare rope, it is less neat than when it is held by roller/stirrups, and it does come round the back of the horse so the horses hind leg action does travel down the rein. I do actually like to use that as an advantage in some ways, for straightening the horses body so that hind end is not swinging out which can happen in circle work in a bigger space, and for asking for lateral movements with the outside rein influencing the outside hind leg directly if you get the timing right. This is my preferred method to start - always in a round pen or secure arena. You should do some prep work first, making sure the horse is relaxed with the feel of the lines. I have a short video about starting the horse off on longlines somewhere on Intelligent Horsemanship website, but I think you may have to be a member to see it and other educational vids (£25 a year).
Your body positioning is crucial as is quick but soft timing with your hands, so if you are new to it and so is your horse I would definitely recommend getting someone experienced to come and longline first for you and get you going. Obviously I am going to mention that this is something IHRA's can do, but there are also plenty of other avenues - some BHS instructors can longline, and lots of classical dressage instructors can too - but try and look at some video of them longlining before getting them involved as there are lots of different techniques and bad longlining can be so detrimental. The thing I think you want to be most wary of is heavy constant pressure down the reins, which is some peoples preferred style - this can not possible be good for developing a soft, happy mouth, so avoid at all costs!
Best of luck,