I have mixed exactly two batches of discharge and on both I took the lid from my ink container, poured a splash of warm water in there (from the coffee maker), tared the whole thing on the scale and tapped out the amount of activator powder I needed. Swirled it around for a second and mixed it in. Worked great, dissolved instantly and not a clump or clod after mixing in.
I think the idea, as with mixing almost anything, is to dissolve the powder first and then cut that into the ink. If you dissolve the activator powder in your lid full of water first all you need to do then is disperse the watery solution now carrying the discharge agent in your waterbased ink, an easy task to achieve.
Or be a lazy arse like Properi-Tees here and just dump it in there.