it's in fact rather straight forward..
two bearings are pressed into the hub (one on each end)
(if they're not pressfit, they're the wrong size or the hub is shot)
when in doubt.. chances are it's
METRIC bearings
if you can name the make and model of your kart we can likely tell you what bearings are to be used.
Then, with the help of some spacers the hub then is slide-fitted onto the spindle
the spacer locates the wheel properly (different widths require slightly different location)
and it adds a counter for the lock nut.
And that's it.. the locknut holds the assembly in place.
(again : sometimes with the remaining spacer to take out the slack of the long spindle shaft)
Now the wheel is mounted to the hub with it's bolts and you should be done.
If you have wheels with embedded hubs, it's essentially the exact same thing..
pressfit bearings into the hub (of the rim.. one on each end.. sometimes with a spacer in between depending on the rim)
slide fit onto the spindle (again with spacers in front and behind to locate the wheel)
locknut.. done!
'sid