First of all you need two routes (E.g. Route 14 and 15)
Go to route 14 (or 15) now make sure they have grass nearby, and you have battled the trainers, plan your way to the grass, make sure the trainers are beaten on the way to the grass. now make your way to route 15, every time it appears on the corner of the screen Route 14/15 check your pokegear, that's when they roaming Pokemon move, now go back to Route 14/15 check it again, Until your persons face has little an oddly shape behind it, that means the roaming Pokemon is on the same Route as you. Now go into the grass, It should be the first Pokemon you encounter, if it isn't you'll have to move back and fourth again. Now, use a pokemon that has scary face, or a move that will put it asleep or Block also the Pokemon also has to be faster than it, (Your pokemon should be since they're not high levels). Once it's alseep or something or you used Block (You must keep the pokemon in battle when you use block), lower it's HP, try use the pokemons weakest move. Once its HP is low enough, use Ultra Balls, or even Dusk balls if its night (I caught Giratina with one, and Mew with a Pokeball in Pokemon Red) After throwing a good few pokeballs it should be shaking at least, if you run out, turn off the game and try again. Saving before the battle won't help you, (I saved before I battled Rikou and I killed it, I turn it off then went into the grass again and Entei appeared) Good luck catching the Entei, Rikou and Latias/Latios!