Most people just add a Chocolate Chip Starfish leg and wait for the shrimp to eat it, but, if you want some specific advice, I'd recommend feeding them daily with two 3 cm pieces of starfish. Let them eat as much as they want and add more the next day after they've finished the previous feed off. Repeat the process. If they don't finish the food within two days, I'd pull the uneaten food and replace it with new food.
If your water quality is good enough and the starfish you're using for feed is of high enough nutritional value to the shrimp, then they should spawn and produce viable offspring on this regiment. I have no idea if Chocolate Chip Stars, Sand Sifting Stars, or Asterina (Aquilonastra) Stars are of high enough nutritional quality, but they're the go to starfish for feeding Harlequin Shrimp in the hobby (Linckia Multifora being the preferred diet of the shrimp for breeding purposes, but they're prohibitively expensive and are harder to keep alive).
Edit: Just to add, they will eat as many Aquilonastra Stars as are in your tank over time, so I don't know that I'd try the two 3 cm pieces part if using them as feeders, but it holds for other types of starfish. If you go with Aquilonastra Stars, I'd say add something like 6-8 at first and watch to see how many get eaten in a day to see if you can't find a good number to feed them daily.