What is the proper procedure for placing the vehicle into 2wd?
If it's working properly, you should just have to push the lever. With hubs locked, you can shift back and forth while moving. No need for neutral, just let off the throttle when shifting. I'd shift between 2 and 4wd on the fly on icy roads. As I approach a stop sign, pull it back into 4 for even braking to all wheels. After accelerating back to speed, push it back to 2wd. Sometimes if it's a little stubborn, blip the throttle as I'm pushing.
This assumes manual hubs, if you have automatic hubs, you have to stop to shift into 4wd.
If you were using 4wd on a high traction surface, then it may be bound up as was previously said. If backing up doesn't work and the hubs won't unlock from being bound, you can also try jacking up the front axle so the tires are off the ground. That should release any bind.