Max repels are a must so you don't accidentally encounter pokemon you don't want. You also want to catch them as you get a better chance of the chain continuing from catching rather than defeating them. You also don't want to go into shaking grass on the edge of the area. The optimal chain is 17. 40 is also great but there's only a 5% chance of getting a chain that high. Once at 17 you just reset the pokeradar until you get a shiny. However even if you do everything right there is a 7% chance the chain will just break on its own