An in-swinger is a spin ball that will go slowly to the batsman. To bowl an in-swinger, you will have to have one shining site on your ball. (rub it against your trousers so it gets clean and shiny) Only one site should be shiny, the other should be filthy.
Now, if you are a right hander, put your index finger on the stitching (the part in the middle of the ball that keeps it together) and your middle finger on the shiny site. Your thumb should be also on the stitching, on the opposite site of the ball.
Notice, the shining site should be on your right and the filthy site on your left.
If you are left handed, it is the opposite way round.
After that it is normal delivery. However, you should get a good run up.