I would add, "Check WARM idling in neutral". Or maybe "hot", I can't remember which.

I agree with much of what's been advised. First, check fluid Low fluid will definitely cause slipping! Needing a fluid and filter change probably won't.

Second, I wouldn't dive into this one yourself YET. Kudos for wanting to, but a bit advanced right now. Just learn how to check the fluid and keep it in operating range, I'd say with Dex/Merc.

Third, FSM's are definitely AWESOME. Find a greasy cheap used one on Ebay

So check fluid with trans warm, idling in neutral on level surface, and yeah, parking brake on, and get back to us