Take your workpiece

Remove any protective film on the top side.

Take a BLACK drywipe (aka whiteboard/dry erase) marker, and scribble all over the area you want to engrave in. Try to ensure good coverage, and be sure to go over any obvious gaps or weak areas - not doing so will affect the outcome!
Any areas not coloured will not engrave, as you will see on the left hand side of the finished item in Step 3.

Important:
Don't use Sharpies or other permanent markers. Certain alcohol based pens can even cause the acrylic to craze/crack.

Place the piece in the F1. I'd recommend using the black cutting block underneath, and set the focus so the dots align on the drywipe layer on the top of your work piece.

Choose your design and check it's framed as normal.

For settings - try working from here:
Blue Light
50% Power
400 mm/s speed
1 pass
200 Lines per cm

Important:
Work up from these settings. Too high a power/too slow a speed can cause black spots/voids within the acrylic from the laser burning imperfections in the material. This may also result depending on whether your clear acrylic is cast or extruded. If in doubt, run a blue light test grid on a spare piece.

Wipe the piece with a dry tissue or cloth to remove the remaining drywipe marker. 
Don't use IPA/Rubbing alcohol as this may crack your acrylic.

Enjoy the frosted engraving!