Executive Summary:
If you plan to fly via Frontier, don't.

Technical details:

The prices are a lie, they will add $79 for a checked bag and $69 for a carry-on, so the flight isn't even cheap.

They will force you to check in on the mobile app, which is fantastically user-hostile. It has a button to add a bag. Click it and it says "Bag Added". As you will find out at the airport, this means you did not add a bag.

At the airport, there are only two lines: one short one for "premier" people only, and a long line next to a big sign saying "No Checked Bags". Both lines are wrong. You need to go around, past the EXIT ONLY sign, to get to the Step 1 stations to tag your bag.

The machine says to scan your boarding pass on your phone, but this is apparently impossible. The only scanning device is a big glass plate intended to read passports, and it will not scan a phone boarding pass. The secret, hidden scanner is under a ledge, totally invisible. I found it by trying many increasingly frantic motions and swearing.

However, now the app has displayed the wrong boarding pass--it's for my flight last week. There is no apparent way to display the correct boarding pass. Finally, through pure luck, I discover that there is an invisible swipe-left motion that will display the correct boarding pass. Now I can discover that I haven't added a bag. I must do so now.

Of course they don't accept Apple Pay. They require me to type in my credit card number and three other fields again. I had to do this on my previous flight a week ago, but the app did not save the number. Why would they want to do that?

Now I can finally go wait for a very long time in the "NO CHECKED BAGS" line. There are stations for a dozen agents, but only two staffed.

Clearly, Frontier Airlines is an intelligence test and I have been flunking. For all that is holy, please be smarter than me and use a different airline.