Put two Gleitlager into the second-biggest holes of each endkappe_kreuzschlitten so that their rim fits into the depressions in the endkappe
Add to each endkappe_kreuzschlitten a Mutter tr8 into the middle hole from the other side. Add Madenschrauben to the tr8 Muttern but don't tighten them yet
Attach two endkappe_kreuzschlitten to each of the Profile with 8 M5x10 screws so that the rims of the Gleitlager are covered by the Profil.
Attach the now u-shaped two Profile to each other so that they hug each other with their u-shape and fix them with 8 more M5x10 screws