Making a Gen One here is pretty easy.
A Custom can have a Background added, something their kids at this time just can't have. Hey, Gen Ones get to be special right :)
1. Go to Make a Custom and purchase the raw Base. Depending on the species and the variety, the cost may be low or even free, or pretty costly.
2. Customs get across the board 25 to start for Skill, Charisma, Show and Value.
3. After you get your raw Base, go to the Bases shop for your Pet, unless you want to keep them as is. Choose a pretty Base and purchase it. Go to My Stuff (inventory) and apply it to the correct Pet.
4. Now, do the same for the markings you'd like. Again, be very careful to apply to the correct Pet. The new Customs will be called Your New Custom!
5. There you go, your Custom is done!! You may always go back and purchase a new Base or change the markings.
You aren't trapped in how your Gen One looks!
This is why it's expensive to start off, you can swap out and alter so much, even the gender!
Be very careful when you choose a Base or Markings to apply, that you have the correct type of Pet!