First of all, you need lots of max repels and poke balls. You should find an area with a bunch of grass. You can't use your bike, roller skates or anything else. You will need to have your pokeradar registered. Walk into the middle of the grass, use your repel, and use your pokeradar.
Once you find your first Pokemon, catch it or make it faint. After that, watch for the grass that shakes. If it has sparkles, then you got your shiny. If the grass shakes slowly, then your chain could break due to a high chances of another Pokemon. If the grass shakes at a medium speed, you have about a 50/50 chance of encountering the same Pokemon. Fast shaking grass has a high chances of having the same Pokemon.
Using the rule above, you will want to step into the grass that is 4 steps away from where you are at. You don't want to go past that or go to the borders. Even if it is 4 steps away and it is slow, don't take it. If there are no good patches of grass, walk another 50 steps and try again. Once you reach 40, your chances will be 1/200. Remember that finding shiny Pokemon will end the chain.
Hunting Pokemon with low encounter rates can be harder. Hope this helped!