First of all, SP is different for everyone, and even the same person can have different feelings every time, so it’s not so sure you will see HI or feel vibrations.
The method I use is the following:
-After a WBTB, when you lay down, find a comfortable position. It’s quite easy to enter SP if you lay on your back and let your arms rest just next to you, without touching your body. This position is usually uncomfortable for sleeping but it’s very effective for WILDing (at least it was for me).
-When you feel comfortable close your eyes and breath slowly. Concentrate on your breaths and count them (this helps you keep concentration).
-If any thought arises, let it go. Do not think about anything that is not your breath. If you find yourself thinking about anything else, concentrate again on your breath. It’s easier if you count up to 10 and then restart from 1; this way the counting doesn’t tend to become automatic and it’s easier to find out if you lose oncentration.
-At some point, if you have done it well, you will feel some vibrations that usually start from the tip of one of your limbs. I don’t know if they are the same for everyone; for me, they’re weak vibrations that start from my left hand, diffuse to all the left arm and then they diffuse to the rest of the body. For some people they start from the feet, and maybe for you they will start from another place. Just ignore the vibrations and stay still. It’s nothhing to worry about. The first times you feel SP it may be scary, but if you know what it is, you have no reason to be frightened. Anyway, as soon as the vibrations arise, start counting in a different way: you should count alternating numbers to the words “I’m dreaming”, this way “1, I’m dreming; 2, I’m dreaming; 3, I’m dreaming…” and so on (the words “I’m dreaming” are not really necessary, but they work almost like a self suggestion). You should count at a rate you feel comfortable with, for example with your breath or very slowly (as wissam told me before). Here you need to be less concentrated, so it’s better to keep counting until you reach 100, not 10, and then restart.
-With these vibrations, HI may arise. Unluckily this is usually not my case, so I can’t describe HI very well… you could see very simple or very complex and realistic light patterns, sounds, or other feelings… they can change over time, for example, you could see some simple HI and after some time it becomes more and more complex. HI is not so easy to pass… you must ignore it, but not too much; if you ignore it completely, it may disappear and it’s like a step back in the process, but i you pay too much attention, it’s easy to lose concentration and fall asleep completely. Think of them as part of the environment, and think of them as they were real. Try to not think “This is just HI” or they will very likely disappear. Just keep counting.
-After some time, you will enter the transition phase. It’s the last obstacle before the WILD, so you are almost there! Physical sensations could change completely. You could feel stronger vibrations or even something weird like people walking on you. Also, HI could become more detailed and complex. Keep acting like before. Remain passive and keep counting. It may be hard to remain passive with people walking on you, but if you lose concnetration at this point, you’ll fall asleep.
-If you are successful in mantaining the concentration, then you have done it! You will find yourself inside a dream! Unluckily, I can’t be of much help here; I WILDed only one time. I suddenly found myself laying in my bed with my eyes open (though I did not open them) but lights were on and the window was open (if I was still in bed they shouldn’t have been, right?), so I did a RC and it had a positive result. Now, how to avoid my error: as soon as you enter the dream, pay attention to the environment. Look around, stare at something (ie. your hands) until you see a lot of details, concentrate on sounds and smells, search for wildlife… try to feel the world around you as it was real. This is useful in any LD, not just in WILDs; it helps improving the stability of the dream… I’m saying that because my WILD was very short… I wasn’t even able to get out of my bed 
Oh, a last note: as Cynster said, if you have an itch, ignore it. If you keep concentrating on the counting, the itch will feel somewhat distant and unimportant.
You have very likely already read almost everything I wrote in this post, but I hope I was of some help 
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