"Primitive" Fire starting
One thing I found is that, for me, it helps if the bow is NOT flexible. I actually used a cow rib bone once when I didn't have my kit with me, and it worked pretty well.
For the bearing block, I found a nice river rock that fit my hand, and I chipped and ground out a socket and lubed it with elk tallow. The most annoying thing to improvise is a good bearing block (this is what you place on top to apply the downward pressure on the spindle and keep it all from wobbling around).
I have improvised EVERYTHING in the above list at one time or another. Normally you can just tell when the wood will work.
The bag under it all is what I carry it in. It is (poorly) braintanned elk. Not shown are 6 extra spindles (one is yucca), a spare fireboard (2/3 the length of the old one), and a piece of birchbark. Sometimes I also keep an extra bow cord in there. I also have a yucca spindle in there with buckskin thumb-loops for more hand drill attempts ("making blisters").
Then you put it all together and do this:
Keep your hair tied back; bandanas and stampede strings out of the way, and keep your body over the spindle, with your shoulder directly above the bearing block and your wrist locked against your shin bone for stability.
Good luck ... or better yet: practice.
Since we are on the subject of fire, here are some easier methods. If you have access to the right materials (a steel striker, flint or chert, and some char), you can make a flint and steel fire. "Thank you" to one of my regular readers who sent me this good-sparking steel!
What if you have nothing but a magnifying glass (or far-sighted glasses)?
And, you can make it even easier by having some "char" to focus your lens on. Funny thing is, the glowing part of the char is hard to see in the bright sun. It catches almost instantly and might catch you off-guard. Watch me burn my fingers and be amused.
Now, if you want a fire really easy, and I mean "matches and gasoline" easy, try a firesteel (from firesteel.com) and some dryer lint. It just doesn't get easier than that. My 5 year-old daughter can make fire in seconds this way (see video at the very bottom)- and has more than once.
There is really no excuse to be without a fire in a survival situation. Even in wet conditions you can find some dry tinder somewhere, and you should be able to find some method to light it. Just learn and practice.
There are more primitive, much more difficult (for me) methods, such as the hand drill and "fire plow" (really "rubbing 2 sticks together"). If I ever manage to succeed with either of those methods, I'll post the video. If anyone wants to come here and teach me those methods, I'm up for it.