This is a really easy way to ensure that you have exactly the same amount of weight in both seatbones.
At a standstill, with someone holding the horse’s head if it is a reactive sort, take your feet out of the stirrups and bring both knees up to touch each other in front of the saddle.
You’ll really be able to feel your seatbones.
If necessary, wiggle around until they feel exactly the same. Then gently drop your legs back down and get used to the new, symmetrical feeling.