Option #1:
Trelleborg carbide studs (or other brands with carbide studs) work good in a variety of conditions but best on snow covered trails. Snow limit is about 8" deep. Beyond that the rear wheel tend to spin too much plus the front wheel doesn't track well. These tires are pricey, but they last several seasons. Tires with custom installed carbide studs run about $200-250 each. Trelleborgs run about $300-350 each. You can also find used Trelleborgs with good life left for about half cost. When you consider the prices of ice screws (see option #2), carbide tires can be economical when you factor in multiple seasons.
Option #2:
Kold Kutter screws (i.e., specific ice screws) have a special cupped-shaped hex screw head. The cupped-shape has a very sharp cutting edge that bites into the ice. These screws are put into the knob from the outside and easy to do with the tires already mounted on your bike. Shorter screws such as 1/2" or 5/8" can be used without tire liners. If you use longer screws than the knob height, then you will need a tire liner. Tire liners help to anchor the screws to prevent losing them during rides.
Tires with Kold Kutters in each knob or multiple screws per knob work great on frozen lakes, very icy trail conditions, and hard packed snow. They can be used in loose snow conditions but the snow limit is about 4" deep. Kold Kutters don't last long and get "beveled" very quickly when riding on frozen ground that is not covered with snow or ice. You will also lose screws occassionally. Tires will last only for 6 to 8 hard rides. Depending upon your season length, you may need to rescrew your tires if the knobs are not trashed. Cost for Kold Kutters is about $100 for a set of 2 tires.
Option #3:
Like HuskyDude shows, there are other creative options using hardware store screws. I've seen folks use carriage bolts, concrete screws, sheet metal screws, etc... pretty much anything that is metal. Concrete screws would be a better choice since the metal is stronger. These type of custom tires are typically done by inserting the screws from the inside of the tire (sharp point is outward). This differs from Option #2 since you are not running on the head of the screw but rather the point of the screw.
Like HuskyDude's pic, this type of tire is better for snowy trails. The longer screws would be better for deeper snow. The downside of this tire is that the longer screws will flex when riding on frozen ground or ice. Also, they can cause a bit of bodily harm trying to mount the tires. I did this once with mountain bike tires and it was not fun... I can't imagine dirt bike tires with a stiff bead. Also, you will need to put a tire liner between the screw heads and inner tube. A easy route is using Duct tape or cloth tape and wrap several layers to protect the inner tube. The cost is cheapest at about $30-$50 for a set of 2 tires. The durability is similar to Option #2.
Conclusion:
If you are just trying out some winter riding, then Option #2 or #3 will be fine to start enjoying this new riding experience. If you plan to do it every season, then Option #1 is a good investment and provides unreal traction. I love winter riding and look forward to the cold season now since I can ride year round.