Birkenstocks are German-made shoes and sandals which feature an ergonomic footbed made of cork. The presence of cork in the sole makes Birkenstocks conform and mould to the shape of your feet for comfortable impact with every step. Because the soles don't initially conform to your feet, you need to allow for a breaking-in period, to avoid hurting your feet and to allow the muscles to get used to the new support. You can break in your new Birkenstocks with a little effort and a little bit of time.
Put on your Birkenstocks without any socks when you take them out of the box for the first time. Walk around your house for an hour with the new shoes, then take them off for the rest of the day.
Put on your Birkenstocks the day after your initial walk and wear them again for an hour. Do this everyday for the rest of the week, both to stretch your muscles and the leather of the Birkenstocks.
Increase your wearing time to two hours per day in your second week of wearing the Birkenstocks. Wear the shoes each day of the second week for this amount of time unless your feet feel sore. Spread the time out over the course of the day if doing it all at once is too much for your feet.
Wear your Birkenstocks outdoors during the third week of breaking them in. Increase the time to three or four hours a day as well. Try to wear your Birkenstocks while doing your normal daily routine so you get used to the feeling of wearing them as you would a well-worn pair of shoes.
Add an hour at a time to your wearing time until your feet and the shoes both feel comfortable. Allow a few days in between adding time if you find the increases still result in discomfort.
Wear your Birkenstocks indoors while breaking them in so you're not out and about in the event that your feet starting hurting. Use your discretion when breaking in your shoes; go faster or slower depending on your comfort level and how your feet feel.
Do not soak your Birkenstocks in water because the moisture can damage the cork of the shoe soles.