Shoutout to the pissbaby downvoting every single informational post in this thread!

Anyway, from Penn Station:

Walk to the St Paul side (that's where the cabstand originates). There's a bus stop with a small shelter just north of the 83 offramp/Mt. Royal intersection.

There will be MTA buses, Circulators, and other private buses that won't do you much good. Take any MTA bus that says DOWNTOWN (or feel free to ask the driver) or a Circulator (it'll be marked PURPLE ROUTE). MTA buses are $1.80 and they don't make change, but it's not exact fare. So $2 is maybe more convenient.

There's an app called CharmPass but I don't know that I'd go to the trouble for a single trip.