You could do an all in one mix like Happy Frog Fruit and Flower, or Down to Earth Rose and Flower Mix, or Dr Earth Bud and Bloom Booster. Following the directions on the box, minus a little.
Or a custom mix of stuff like crab shell, kelp meal, bat guano and gypsum. Like a teaspoon or two of each.
One guy has been getting great results with economically priced Plant Food Spikes from the hardware store.
Whatever you decide, it's important to be ready. Half way through flower your plants will be bigger and hungrier, its common to get yellowing from being deficient in phosphorus, potassium, and calcium around week 4. Top dressing can take a week or so to start working, so you don't want to wait too long.
