The nurseries sell ladybugs every spring to release. Their instructions are to overhead water so the foliage is wet because the lady bugs will be thirsty after being stuck in a box all winter. Continue to provide a water source, like a shallow saucer with a paper towel in it.
Release them in the evening near the aphid infestation. If they find somehow your accommodations to be inadequate, they will fly away. Thirst seems to be the biggest reason they fly away immediately.
