In order to begin building foot strength, all you really need to do is spend more time out of your shoes and let your feet do their thing. This is why shoes are often referred to as foot casts.
Once you start spending more time out of your shoes, you will notice that your body responds better to barefoot movement. This is what I would call the beach syndrome. You spend your whole day lounging and walking around at the beach sans shoes and then when its time to go home, you put your shoes back on and it absolutely sucks!
To take your foot strength to the next level, it's important to incorporate specific movements and exercises that target the feet and ankles. We have a ton of resources and exercises you can utilize, but one of the easiest other than simply walking around barefoot is the Standing Ankle Push-Outs.
Standing Ankle Push-Outs are great for building overall arch strength and perfect for people with flat feet, eversion, or pronation. Your goal should be to get a couple sets of them in at least 3 times per week.
Check out the video below: