Regional Day/Lander-Ticket on DB Question

1) Do I need to buy in advance from home? Or can I buy at the Munich hauptbahnhof when I arrive, for travel a day or two later? Or, buy on day of travel?

You can buy a Bayern ticket at any DB ticket machine in Bavaria. You can either buy it for "today" or post-date. You can also buy it from the manned ticket counter, but there is a small surcharge for doing so.
You must write your name on the ticket.

2) Do I need to reserve a seat or just check the schedule for the train I want and board a 2nd class car? It seems I can't take an IC train, what designations will I be looking for on the schedule?

These Länder tickets are valid on any regional trains, that is anything except IC, ICE and EC designations. Also valid on U-Bahn (Metro), trams and buses. The designations for these trains are: R (=Regional, stops at all stations), RE (= Regional Express, doesn't stop at every station), IR (Inter Regional), S (=S-Bahn = Commuter/suburban trains), plus some more for trains run by other companies.
IC, ICE and EC are the only trains that have reservations, no reservations are possible on the trains on which the Länder tickets are valid. Just get on and find a seat. Many regional trains are 2nd class only, no 1st class.

Here is a map showing all the rail lines, and most of the stations, where a Bayern-ticket is valid: https://www.bahn.de/p/view/mdb/pv/deutschland_erleben/allgemein/tickets/pdfs_laender-tickets/2015/mdb_190311_streckenkarte_bay_2015.pdf

Thank you, Chris--exactly the info I need!

Giengen is in Baden-Württemberg, so a Bayern-Ticket will not take you all the way there. It looks like the best way to get from Munich to Giengen is by way of Ulm, which, although also in Baden-Württemberg, is a border station for the Bayern-Ticket.

In addition to the Bayern-Ticket, you will need some sort of ticketing to Giengen. Giengen is in the HTV (Heidenheimer Tarifverbund) but is in a tarif area which is shared with DING (Donau-Iller Nahverkehrsverbund Gmbh), which includes ULM. From the DING website, a Tageskerte Single (day pass for one person for Ulm to Giengen) will cost 13,70€ from a ticket automat in Ulm and will include RT between Ulm and Giengen.

So for the trip to Giengen, a combination of Bayern Ticket and regional Ulm area ticket would be €38.70 whith a 9am restriction and a travel time of 3 hours ore even more, while a saver fare ticket for the ICE booked today is €39.80 with a travel time of 2 hours. I'm pretty sure that my extremely frugal grandmother would have taken the Bayernticket; myself I would probably be so light-headed to choose the shorter travel time for the hefty supplement of €1.10. ;)

For the trip to Nürnberg a Bayern Ticket is a good choice provided you take one of the RE's which run over the direct ICE line (aka München-Nürnberg Express), the first one leaving Munich at 9:01 (RE 4674). Never board a RE that goes via Augsburg and / or Treuchtlingen; they will take up to 3 hours.

I think a regular train ticket for Giengen will be fine even if it costs a little more--I'm only going there to see the Steiff museum and that's apparently a short walk from the train station so the benefit of being able to ride a bus, tram or metro won't be needed like in Nurnberg.

I appreciate all the info, for some reason the one thing I get anxious about planning is train travel, even for a day trip.

the one thing I get anxious about planning is train travel

There is no reason for that. Tell us when you want to arrive in Giengen and Nürnberg and we can give you a step by step instruction (it's not complicated at all).

the one thing I get anxious about planning is train travel, even for a day trip.

The thing about trips by train on regional trains is that they don't need much planning, not much more than a trip to a shop in the next town you haven't been to before.
If you're a map person (like me) you can look at a map first and see "I need to go north to xxxx and then on to yyyy", or you can just go to the railway company website ( https://www.bahn.com/en/view/index.shtml ), plug in your trip details and see "Yes, trains at 08:45, 09:45, 10:45, etc., change at xxxx, then a local to yyyy". Then head down the station and look for the train to xxxx at 45 minutes past the hour.
Coming back, no stress, you already know there is one train per hour leaving yyyy at 17 minutes past each hour (you looked that up). So when you think it is time to go back, just wander back to the station. get on the train, and you change at xxxx, same as you did coming.

I think the anxiety for the Giengen trip stems from having to make an 11 minute connection to another train in Ulm--for example, arrives at platform 1, connection departs from platform 5A. My previous trips in other European countries have all been direct. My fear of getting on the wrong train has improved--I check with the conductor or other passengers.