While it's not the most efficient thing to do, it beats postholing in the snow--gather up a (hug-sized) bundle of evergreen boughs. Separate these into two bundles and then tie them securely using 3-4 lashings per bundle. These can then be tied onto your boots and used for short distances. They will collect snow and get weighed down, requiring frequent "cleaning" if the snow is especially wet. Again, this method is not very efficient but beats the alternative.
Some of the methods that show up in survival manuals would have you believe that it's as simple as bending a flexible pine bough into a hoop and then lashing on a cross-member of sticks. All I can say is that there's a reason the native cultures of the far north had wide, elongated snowshoes--because you need surface area on your snowshoes to spread out your bodyweight. A small hoop won't work.
Good, field-expedient snowshoes, that I have made and used on a winter survival course in the sub-Arctic using spruce, took me almost a full day to make--and that was when I was well-fed, rested, and without injury-- something you may not be able to say when you're stranded in the backcountry during nature's most challenging season.
Best thing is to plan ahead if you know you're going to be in the deep powder--remember, outdoor safety and survival is about being prepared. Bring a lightweight pair of aluminum snowshoes with you when venturing into the wilds during the winter. These can be strapped on the daypack until you need them. While I am a sucker for traditional Bearpaw-style snowshoes made from steamed ash and laced with rawhide, the lightweight models out there today are pretty nice and will sure make hiking easier (and more efficient).
If you are caught out in deep snow without snowshoes and the sun is getting low, it may be better to stay put, conserve your energy, and build a shelter and fire for the night.