Guide your horse next to a mounting block. From the block, push down and lean into the saddle with your hands, allowing it to feel your weight. You may need to try this multiple times so the horse is used to having weight in the stirrups without moving away. Mount your horse and sit calmly in the saddle for a few minutes before dismounting. Knowing how to mount a horse properly will help you as a rider, and help your horse get used to the sensation. Your horse will get used to the motion of being mounted and dismounted. It will also get comfortable holding the weight of a rider. Remain in the saddle in short periods of time and don't move. This will build their strength and prevent injury to joints or muscles. When you're in the saddle, gently squeeze your horse with your legs to begin walking. If it doesn't begin walking, you can gently tap it with your heels. Low-impact, low-speed exercise done in sessions of 30 minutes to 1 hour are best for your horse’s training and health. Your horse is ready for walking if it does not flinch or resist when being saddled or mounted.

Summary:
Use a mounting block to climb in the saddle. Mount and dismount from the saddle in 10-minute intervals. Begin walking your horse while in the saddle.