The PWM frequency is not completely determined as you hasn't provide what frequency you are using for the ATtiny85 main clock.
The code below assumes that it is running at 16MHz using the built-in PLL.
In that case, this code uses timer 0 to provide pins 5 and 6 with an 8-bit PWM output of approximately 31kHz.
// Timer0 setup
DDRB |= 0x03; // PB0(pin5) and PB1(pin5) force OUTPUT
TCCR0A = 0xA1; // Use both OCR0A and OCR0B output, Phase correct PWM mode
TCCR0B = 0x01; // Prescaler not used
// Output value
OCR0A = 0; // This register controls the OUTPUT of pin 5
OCR0B = 0; // This register controls the OUTPUT of pin 6
/*
set to 0, outputs continuous LOW
set to 255, outputs continuous HIGH
set to between 1 and 254, outputs PWMs for each duty at about 31KHz
*/