New Part and Relationships

The design is putting me a fight but I wont stop. Today I started designing the floortray and learning, using some tools to drill arround de border line of it, in order to use rivet to add it to the kart when I build it (someday haha).
Also experimented with some relationships betwen parts in an assembly. I have the doubt if should I relate faces or plans?? It is said that to relate plans is quite better, but is getting harder for me because sometime the round pipes of the chassis confuse, but I wont stop finding out
Thanks for reading