Connect your programmer and start Arduino.
By default ATTiny runs at 1Mhz. To use the addressable lights, we need to set the fuses to 8Mhz or 16Mhz.
In Tools -> Board, select either
ATTiny85 8Mhz (Internal Oscillator) or ATTiny85 16MHz (Internal PLL), then in Tools choose Burn Bootloader (you'll only have to do this once per chip).
WS2811 pixels work with ATTiny at either speed. 8Mhz is fine for most things and will save you a tiny bit of power. The 16Mhz profile has brown-out detection enabled and will cut out at 4.3V. If you plan to run your lights at less than 5V, always use the 8Mhz profile. Read more about microcontroller fuses and brown-out detection.
Once this is done you can try uploading a sketch to make sure everything works correctly.
Choose Examples -> Basics -> Blink
Change the LED pin from 13 to 4.
Connect a simple LED, the positive end to digital pin 4 on the ATTiny and the negative to ground.
Upload and you should have a blinky LED!