I tried it some time ago and I found it quite difficult.
The result was very hard and uncomfortable with unpleasant edges.
The theory is: make an imprint in alginate, fill with silicone, put a vibrator in.
The first challenge is: Get a boner and press it into a tube with alginate. A boner, latin: erection, stands erect. How can you press an erect boner into a tube with fluid or semi-fluid alginate? Without touching the outer walls...
Try going to the toilet with a boner and tripple the difficulty.
Maybe it would work if you have a little help from your friends and a sex swing.
Furthermore in the instruction was a hint to use a cock ring. Strangely cock rings don't work for me. I had a silicone one by the fun factory. It was too tight and hurt like hell.
After that you fill the casting mould with silicone for about one half and press the vibrator into the silicone. You have to hold the vib exactly into the middle of the mould, otherwise you will see it in the result.
After hardening you draw your clone out, have a close look at it and throw it into the garbage can.
It is horribly hard, has sharp edges and looks like a zombie prick.
Have a great night with no nightmares!