Counting is probably most effective when you have trouble falling back to sleep after a WBTB because of distracting thoughts. The counting gives your mind something to focus on so you block out the distracting thoughts. If that's your situation, then give it a try. Otherwise, you might have more luck with visualization or SSILD. You could also use counting + breathing exercises as a method to relax, then transition to a different technique for induction.

Count in your head. And yes, you would continue this until you fall asleep if you're try to DILD or WILD. If you're just trying to relax, you would stop at some point. Counting backwards from a 100 might be more effective because it takes slightly more concentration so you're less likely to slip into auto-pilot and become distracted. You can also count backwards from 300, skipping 3 at a time (300, 297, 294, 291, ...) to make it even more sticky. Don't worry if you lose your place. Just start again near where you think you left off or from the beginning. (I learned this from the book I Can Make You Sleep by Paul McKenna.)

You should notice some change. Body sensations, hallucinations, feeling of sinking or falling. Or you just might start to feel very heavy in your body or a slight high or lightheadedness in your head. Remain calm and let the sensations develop for a while before you move to RC. You're looking for a transition that comes some time after the sensations. Often, the transition will be very obvious, like travelling through a tunnel to a new scene. Then you know you're dreaming and can RC. Sometimes it's more subtle. It might come like a snap of calm in your head like someone suddenly hit mute on the world. Or it might be a woosh feeling, like a wave of sedative medication hitting your brain. Those are a bit trickier to catch reliably. If you sense that, you can RC. Try an RC where you don't move, like trying to levitate out of the bed. Or open your eyes and see if there is anything strange about your bedroom.

Where you end up will differ for each person and each experience. In this method, there's probably a greater chance you will end up in your dream bedroom. If you instead use visualization, you're more likely to enter a new scene.