Electronic – Questions for a personal LED/ATmega328P-PU project

Ok, so first of all, here's I'm wanting to use in this project

• x14 5mm Red LEDs
• x14 Resistors (1 per LED due to individual control –
  how many ohms each? I don't quite understand Ohms law)
• x1 Battery and holder (was thinking a button
  cell battery if possible)
• x1 Latching button (on/off)
• x1 Momentary button (switch modes)
• x1 "Joule Theif" for 3.3v/5v (i.e. https://www.youtube.com/watch?v=K53beWYdIpc)
• x1 ATmega328p-pu (that I plan on underclocking to conserve
  power – there a more efficient chip to use in this situation?)

Each LED would be connected to a separate pin on the chip with a resistor to turn on/off individually to form patterns. I'd like to limit battery use to a single battery (ideally cell for its size, or AA/9v since they're also common). I'd be using 2 proto boards on standoffs, one with just LEDs, the other with everything else and battery/buttons on the back. The positive LED terminals and ground would go into female headers on the other board for easier disassembly.

I'll be completely honest, I have no clue where to start. I'm really fresh into electrical work/arduinos and I spent a good 2/+ hours just trying to figure out if I could get the step up + >3.3v battery method would work, let alone if a button cell battery could do it.

Some things I think I've gathered:

• The ATmega328p-pu can work off of 1.8v minimum, but the Red LEDs would require a higher voltage than that (what would the difference be in Voltage needed in series vs a distribution like I mentioned?)
• A button cell battery probably won't work because of limited mA
• DC/DC Step-ups aren't optimal(?)

I'm willing to go other routes entirely, such as just using an Arduino Nano and just plugging it in for power, but the battery route would be awesome if it could hold 1-2 max batteries and be small enough space-wise. If using a step up, would it be best to aim around 5v instead? I'm open to any and all suggestions. I know there's plenty of questions and all of them are pretty amateur but I find myself in over my head in most relevant posts I've seen so far.

Thank you so much in advance for your help!

Edit: Done some more hours of research and wrapping my head around all this and I have a complete setup designed now, all that's missing are values/power ratings I'd need and some other questions I'd have. I might post those as a new question instead down the road – thanks for your guys' help so far!