To see which litters are available to custom into, go to the plot recruitment subforum
and it's the first link. HERE~. So read the rules there.
Go to the last page and you'll see people posting images of the parents,
how many cubs they're allowing to be custom'd in, certain requirements they may have,
and if they want your custom form to be posted there or PM'd to one of the owners.
If you're chosen, the owners should PM you and let you know. Then you post your custom cub form in the colorist's (whichever colorist colored the littler you're customing into) breeding forum.
A custom cub is 500k.