The Standard of Perfection (SOP) is useful, but some of the terminology can be confusing. I learned best by seeing and feeling rabbits in person. If you have a breeder near you who could be your mentor, that is the way to go.
I raised Harlequins for a *very* brief time. They are posed in commercial type, similar to a New Zealand which I currently have:
Here is the above picture with some lines so I can try to explain what you are looking for.
We'll work from head to tail. The head should be blocky with proportionate ear length. The ears should have a strong base, they should be thick and well furred (can't see skin on the backs of the ears).
The arrow shows a gap. This rabbit is a tad bit long in the shoulders. You don't want a lot of space between the back of the head and where the back starts to rise, approximately 2 finger widths is too much.
The lines at the front and rear feet show where the feet should be placed. The front feet should be under the eye and the rear feet should be aligned with the hip. The line going all the way top to bottom on the rabbit shows the depth. You want a deep body. This rabbit could stand to be deeper in the shoulder also, although he is better than some of my own rabbits.
Now, look at the hindquarter where I drew 2 curved lines. The red line follows the rabbit's actual shape. See how it tucks in at the bottom? This means the rabbit is "undercut" and lacks "fullness" in the hindquarter. You can also see how the rabbit's rear toes stick out some, he is lacking width in the hindquarter -- you can more easily observe this by flipping the rabbit on his back on your lap and examining the natural positioning of the feet. Wide and good: | | Pinched/Narrow: \ / (the feet form a V).
Back to the lines, the black line shows what a good hindquarter would look like. In a proper pose, you can look at the rabbit from above. You want the rabbit to be wide from shoulder to hindquarter.
Additionally, Harlequins are a marked breed. This means they have to present certain markings (color alterations) to be showable. I don't remember the minimum required markings, but I believe you can find decent diagrams online.
** Disclaimer: In my experience, many breeders and even rabbit judges don't put much stock into the conformation/type of Harlequins. I have seen judges choose winners based only on markings. I disagree with this practice, but what can you do?
I hope this helps you!