For years I have been trying to explain Deweyan temporal and processional concepts, and recently I have expanded to process concerns in general. I came to “process philosophy” as the Whiteheadians understand it only recently. It can be very difficult to explain the core of process philosophy in general terms to someone who has not even considered or read alternatives. Reading Spinoza and Leibniz, by the way, does not count as “process” in this respect, since they are quite different from modern notions, e.g., differ on the reality and nature of time, relations, chance, possibility, identity, etc.
Below is another attempt to sum up the difference in a few words; this is something that I would say to curious philosopher.
Substance: a thing is what it is regardless of its or other’s activities
Process: a thing is what it is because of its and other’s ongoing activities.
There are two key differences. One, in a processive view, ongoing activity is essential to the things being, but this also implies that becoming and time is essential. Two, since anything is acting and being acted upon, and this is part of its being what it is, then interaction or transaction is fundamental to what a thing is. Nothing can be fully defined in isolation to its situatedness, because these are also the conditions of its being.