Step 1: Mount the Raspberry Pi to the base
You have to mount the shorter standoffs on the side with the power/HDMI/audio connectors, not on the side with the 40-pin connector.
Step 2: Mount the longer spacers onto the HiFiBerry DAC+
The longer spacers will be mounted on the side with the 40-pin connector.
Step 3: Mount the HiFiBerry DAC+ onto the top of the case
Step 4: Snap both parts together
This step is a bit tricky. Make sure that top and base are aligned correctly. The HiFiBerry DAC+ should now slide into the 40 pin GPIO header.