Start from the PE 4 straight plank position and then slowly lean in as far as possible.
PE 5 is performed with upper feet support and not on toes which allows you to adjust lean. A slippery floor or a sliding object upon which you can place your feet is recommended for optimal performance of the planche lean.
Planche lean is performed with shoulder blades protracted and depressed. This means that you should feel pressure in your serratus anterior and also a bit in your lats. If you do not chances are you not doing the position correct(ly enough).
Pelvis position is posterior which means having the back as straight as possible.
For mastery wrists must be in front of the hips
A mirror is immensely helpful because you can adjust your form while not disengaging your position. Otherwise pictures or videos to evaluate afterwards are good too.
Common errors are piked hips, bent knees, not enough protraction/depression and bent elbows.
It is acceptable to not be able to lean all the way in the beginning while maintaining a perfect lean. Most often people lack shoulder strength, body control, wrist strength or wrist flexibility to be able to increase the lean any further. The wrist work in H1 can help the wrists greatly, a warm up set of wrist work could also be good. For shoulder strength there is no other way than running through the minicycles until you can lean further.
Planche leans focus on building trapezius, serratus anterior and anterior deltoid strength along with wrist strength and flexibility. Triceps and pec minor along with the lats and abs are also focused but to a lesser extent.
As much as the planche lean functions as strength work it could also be argued to be skill work. My recommendation is that it is treated as skill work until you are sure you have good enough form to build intensity with the templates. Then following the templates for mastery afterwards.
The planche lean should be a part of your routine until you literally can lean in and lift your feet off the ground into a slightly piked full planche. After the mastery requirements there is still more lean to be gained. But it turns into skill work, so put your focus into other sPL PEs for building strength.