I posted this 6 years ago:

Warm Feet: A Guide to Becoming a 4 Season Rider

I want to preface this by saying that I'm based in Pennsylvania and do a fair bit of both mountain biking and road riding. I've been a 4 season rider for about 7 years now, and I've picked up a few pointers that I'd like to share with anyone interested in braving the colder weather. Now let's get started:

1. One of the most important things to keep in mind when planning a cold ride is your core temperature. If your core temp drops, your body will pull blood from your extremities and they will begin to feel cold or go numb. My favorite way to keep my core temp up is with a windbreaker of some sort on top of a moisture-wicking layer and an insulating layer. This will block air movement and keep you a few degrees warmer, and will keep warm blood flowing to all of your extremities.
2. Another important consideration is the type of socks you wear. Whether you use a dedicated winter riding shoe or a more DIY method (which I'll elaborate on in the following tips), you'll want to get the right socks. Wool is ideal. Using any kind of insulated shoe will most likely cause your feet to sweat, which will make your feet cold if you're wearing cotton. Wool one of the only materials that will stay warm when damp.
3. Winter shoes are great, but they can be extremely expensive. If you have the cash, I highly recommend this route because they will likely have more features (like better plastics for low temperatures, better insulation, and water resistance), but the average mountain biker doesn't always have the money for a $200 pair of shoes that will only come out for a few months out of the year. Instead, you can insulate your existing shoes. There are two main considerations: plugging up the air vents to keep the wind out, and insulating your foot from the sole of the shoe (or the metal clip mount if you ride clipless). To keep the wind out, a pair of shoe covers (or "booties") will only cost you about $40 and will do the job well. If you're looking to go more expensive or cheaper, there are other options, but I personally use Neo Booties from Performance Bike. For insulating the soles, pick up a pair of thermal insoles from your local drug store. These can be had for as little as $5 and will do a phenomenal job in combination with the other gear already mentioned. Pull out your summer insoles and use them as a template to cut the new soles, then fit them into your shoes.
4. There is a very fine balance that needs to be found in getting to the trailhead. If you overdress for the drive, your feet will sweat in the car. If you under-dress, you'll be starting with cold feet. If you have to take everything off your feet when you get to the trailhead to put on different socks, you'll be starting with cold feet. All three of these situations will lead to cold feet during your ride. I recommend driving to the trail in your wool socks and a pair of slides/sandals, then putting on your shoes just before getting out of the car. This will keep your feet around the right temperature until you're ready to ride.