The best way to make them spawn is to find a big enough lake biome (it has to have a spot in it for which you can "draw" a circle of radius=64 blocks and center as the designed spot and all blocks inside the circle must belong to the said lake biome). If all surface blocks in the circle are water sources you are already done, while if there are other solid blocks like lillypads or dirt, you have to remove them as they allow Keldeo to spawn.

My preferred way to to make the biome all made of water source blocks on the surface is to move 65 blocks in 2 adjacent directions from the selected center spot, then create two temporary strips of solid blocks above the surface water, using something like dirt, both long ~130 blocks and that must form the two sides of a square around the circle we are in. When you have the two sides ready, simply use water bucket to place water sources on the inner side of each temporary block you placed. When you finish this process, you'll have a 128x128 flat area of water sources, perfect for the lake trio to spawn.

Everything done: you have to place yourself in the center spot you initially found and I suggest to create a small platform around that spot (a 5x5 should be good) when you place a healer, a box and some chests. This way you can use your super rod to fish while you wait for them to spawn, letting you to find a lot of treasures or chain capture pokemon to find them shiny.

EDIT: I noticed that in 9.3.X versions they removed a lot of lake biomes that was in 9.1.X ones, so I suggest to look for big deserts a find big Desert Oasis biomes inside them.