When we did it, I went in really, really early (she usually woke around 35ish min - so I went in at 20) and just watched for a couple times - to see what happened and when it happened as she woke up. Then once I saw a pattern, I went in at a better time (around 30 min) and then as she started to wake up, I'd put my hands on her ("holding" her in the cot), as she kept waking, I'd start shhhhhing, then patting. I had success about 50% of the time (where I could get her to go back to sleep), the rest of the time, she woke. But....after a couple of weeks, that was enough
I never could get the amount of "waking" right - I always did too much or not enough, so doing what I described worked better for us.