After reading a book by Robert a Monroe i also found some things to be specific and helpful. You should get the book all explainations are at the end. He says you should relax during the vibrations which will last about 5 minutes. after that you should slowly (mentally or physically?) strech your left or right hand in the direction it is pointed untill you feel what it obviously out of reach. then push it more, your hand will go though it.
Sorry that the best i could do. The book i found helpful was staightfoward otherwise as it didnt read through the lense of 'chakras' and such. I Dont remember the name but its the Prequel to Far Journeys
The only problem was the vibrations chapter was unspecific for me, and i couldnt find any specifics online, so if anyone can tell me how to get vibrations after relaxtion in detail, id appreciate it. i though i knew it but i dont hear roaring sounds ect.