I just add them as i do each wash... I don't need to add them to my DH's business shirts load because they don't get very dirty... mainly add it to the kids loads that are heavily soiled.

Also the OMO used to come in big buckets ... I store the powder in that so it's moisture proof... important when you need to store it for several months. I make the transfer as soon as I open the bag then chuck it out.

ETA: make sure you buy homebrand vinegar to keep down costs.