My wife lays flat on her back on our bed. I usually start by massaging her legs from thigh to ankle. I can tell how effective I am by the sounds she makes as she lets go of tension. This is time well-spent as the more deeply relaxed she is, the better the Karezza.
Then, from the foot of the bed, I slowly move forward on my hands and knees, over her body, until our chests are matched up. I hold the bulk of my own weight on my knees and lower arms, but press into her to the degree that's pleasant for both of us. Gentle movements against her pubis, while our chests are together, soon has her squirming around underneath me. Sometimes, in a gentle, playful way, we will dry hump like a couple of horny teenagers.
No attempt is made at PIV until she says to me in her sweet voice, "Honey, I'm ready." Early in our marriage, rather than me always asking her if she was ready, we decided to have her ask me specifically to put my penis in her vagina, but only when she was completely ready. This let me know she was ready, and just as important, her asking me to put myself inside her was very good for her emotions because her initiating gave her more of a sense of freedom and choice.
Sex had been so traumatic for her in a previous relationship that she had spent a lot of time running away from it, and then giving in to sex she didn't want out of a false sense of duty. Her specifically asking me to join our bodies in lovemaking has been an effective remedy for the trauma, stress, and lack of freedom in her past.