Not totally safe, but if you aren't careless then its really not that dangerous. You can search on google how to jelq, you are stretching your dick with your hand slowly over many months. Of course google will explain it more. And no it doesn't make your dick skinnier because you can also jelq in a different way for more girth as well. Dick size is not something to worry about unless you got literal 1.5 inch death-tier hopeless dick.
Few people know about it, it takes a long time, most men have perfectly fine dick sizes, they don't care enough, everyone assumes all dick-growth techniques are scam (just like how pills are for everything), etc. And its technically not 100% safe but if you really care about it then you should just go on ahead and do it. Nothing is perfectly safe anyways.