Bicycle Tail Light project
Break down your project into steps, both hardware and software, and tackle a step at a time.

First do a block diagram of the system, showing the different components, the layout of the control buttons, etc.

Then map out the desired behavior, perhaps using a flowchart. What should it do when you first turn it on? How will it respond to different button inputs? Will it continue to flash in the current pattern indefinitely until there's another input? Will there be a cancel button?

Then read up on how to power and control NeoPixel LEDs. I believe they use I2C? Are there any NeoPixel libraries you can use?

You might then try using a circuit simulator to create a virtual prototype of your circuit. Figure out how big a battery &amp; voltage regulator you're going to need. (If you're running off of batteries you should use a switching or "buck" type voltage regulator rather than a linear voltage regulator since they are much more efficient. Just make sure you pick a regulator that can provide enough current for all your LEDs plus your Arduino, with a little extra left over.))

Once you get a virtual circuit working, it's time to use real hardware.

For a bike tail-light, you should think about how to make it weather-proof. are there waterproof enclosures you can use? What about waterproof NeoPixels? If there are no waterproof NeoPixels, what about waterproof enclosures with clear covers?

Buy the Arduino you're going to use, and at least a few of your NeoPixels. Work out how to control and flash a single LED. Then gradually add complexity to your project until you get it working.