Wash and deep condition it. MTG and finger detangle then brush it till dry. Loose braid and wrap it up then vet wrap it up below the tailbone and leave it for months. Repeat as the tail bag gets ripped or as needed for showing.
I have a spray bottle with conditioner and water in it and every time I groom my horse I put some on a sponge and wipe it on going in the direction the hair is growing and get a soft brush and brush it down and I also put it all through her tail because it Menes it smell nice it soft shiney and its easier to brush
In the saddlebred world, we strive to have long thick tails. At my barn, we brush the tail, being careful not to put hair out, wash it, brush it again, and the braid it up. We then roll it neatly and fit it into a tail bag.
I used mtg (mane tail growth) and if possible I only brush with tail with my fingers to prevent the brush from pulling everything out. A more exspensive but works amazing is bio mane! Another suggestion would be to keep it in a tail bag with some tail growth of some sort
The most important thing is keeping tangles out so that the hair doesn't break or get ripped out.
I had a foal eat my horse's tail off and I used Mane'n'tail conditioner in some water and sprayed it all over the tail, then took healthy hair and sprayed that on the dock, and put a tail bag on it! Good luck!