I use power heads or water pumps to mix everything. Mix it (salt, Mg, whatever) and let it swirl for a day. Five gallon buckets are our friends.
I found Instant ocean has tons of mag in it already, much more than sea water, so water changes are all that's needed when using IO.
I stupidly bought BRS mag supplement the same time I purchased the mag test kit. Now I'm stuck with mag supplement I'll probably never use.