make sure they are working under themselves with lots of impulsion. make sure your elbows have plenty of give. slowly lower your hands to about knee level and open the reins up, keeping slightly more pressure on the inside to encourage proper bend. stay soft and have a very light hold on the horse. keep pushing forward. This takes a while to master! it takes a lot of muscle to hold a low headset consistently
Slowly release contact, push your hands forward and keep your leg on. When he drops, praise him
Don't just drop your reigns, slowly relaease the contact and keep the same feeling/weight in your hands the whole time until they eventually lower completely. Some horses you have to give first then they will search for the contact, others will ask to lower themselves and then you can give
loose rein your horse at a long walk so when they are relaxed and stretching their head drops below the withers
They first have to accept the contact and when you have a loose rein they should drop their head down look for the contact. At the same time they will be stretching.