See also What is the meaning of the layers in pcb_new and in the footprint editor? (KiCad 5 and earlier) and Full KLC - Library Conventions | KiCad EDA. And of course you can look at the footprints in the KiCad library to see examples.
Maybe Tutorial: How to make a footprint in KiCad 5.1.x? is useful.
By default SMD pads have copper and mask opening. Look at the pad properties dialog carefully.
They are of different sizes. 1206 etc. are not necessarily exact sizes when the LEDs are described. Even if the outline is exact, the pads may differ in size. Height matters, too. Therefore the ideal footprint for some LED may be different than a standard footprint. What is good enough for you depends on the assembly process etc.