I traced my birth Mother a few years ago, it is the best thing that ever happened to me. We have a wonderful relationship and I now have siblings and other family.
I went about it slowly and carefully, sending a letter giving my DOB, place of birth and birth name and asking if she was related to me.
The ball was in her court totally, I posted the letter on a Wednesday afternoon and she called me on Saturday morning.
But I have a dear friend who traced his birth Mother, flew across the Atlantic and just knocked on her door,,,,, she told him she had not wanted him then and did not want to know him now. He was devastated she just slamed the door in his face.
It can go both ways I think the soft or agency route is best.