When tightening a screw, firm will do. Do not keep tightening until the screw can no longer turn because this can damage structure, actuators, and eventually the screw itself. Simply secure the screw to the point that the pressure is firm.
The side with the small ribs will always face towards the extrusion when assembling, so that the ribs can help square the bracket to the extrusion.
When securing a bracket to extrusion it is recommended to pre-load the bracket with screws. When the screws are loaded on the bracket before it is even brought near the extrusion, it is less likely that screws will fall out or get misaligned. The bracket can then be slid into place along the slot.
Building “stud up” is recommended with the REV Robotics 15mm Build System. This means building with the screw head inside of the extrusion. Pay attention to the direction that screws go into the brackets, only the screw head will fit in the extrusion.
When using the nyloc nuts, make sure the beveled “nyloc” side is facing out.
Have a plan for where the brackets and other structure pieces will go before building. Once pieces of extrusion are connected together it can be difficult to go back and add things without having to disassemble the robot. T-slot Screws can be used occasionally, but it is not recommended to build the entire robot this way.