Connect Trinket's pin #0 to the rotary encoder's "A" signal pin.
Connect Trinket's pin #2 to the rotary encoder's "B" signal pin.
It's that simple! If you want, you can put it into a little box, or make a desk stand for it, or mount it on your PC tower, be creative.
These encoder signals will be "active-low". Each of these signals is a switch inside the rotary encoder. Active-low means the other end of the switch is connected to ground, such that when the switch is "closed", the pin value will read low. We will be using the Trinket's internal pull-up resistors so when the switch is "open", the pin value will read high.