Do ground work with them like leading,lunging,freelunging, and liberty.You could teach them tricks like smiling,following you,etc.Give them treats!Grooming them or even just spending time with them in the pasture or stall.Hopefully this helped!
Spending time with them is the only way.
Sit by their stall, graze them, groom them, run around and play, even simple training is bonding just bring them into the arena and make them follow you in circles and around poles and stop and back up make sure you have treats! Any time spent with your horse will build a bond
Horses are herd animals so you want to be part of the herd. You can be with them in the meadow, just sit there with a book. And which is a lot of fun! Let them loose in the arena and play. Jump around and run, stop and run. Make them run with you. You will see they will start playing with you :) don't worry about looking goofy while doing this you are creating a great bond with your horse!