Currently I don't believe they're accepting new breed campaigns? But to start a campaign, you need to open a club using a club key which costs 500 ingots. Then in the club there's a page where you input your breed percentages, and the breed name and submit it for consideration to be a campaign. 

@ChamFox

I have a club for appendix quarter horses but no one has joined 

That's likely because the game already has a club for Appendix Quarter Horses. You can reuse that club for a campaign if you want. At the bottom of the settings page of your club would be where you submit for a crossbreed campaign. If it's not there that just means that the option is disabled currently. 

@ChamFox

I went and checked and someone joined but I can rename my club right?

Yes you can rename your club, that would be on the settings page for the club.

@ChamFox

Thank you and I renamed it

@ChamFox

Now that I changed the name of it what do I need to do next 

Next steps are:

• Go to the setting page for your club and scroll down to the “Horse Registration Guidelines” section.
• Click the “Add Guideline” button.
• Select “Breed Mix” from the dropdown menu which will bring up the “Allowable Breed Mix” section.
• Click “Add Breed” and fill in the percentage of your first breed in your crossbreed. Repeat this step for every breed in your crossbreed.
• click “Add Guideline” and then click “Update Club”
• Then scroll to the bottom of the settings page and there should be a “Crossbreed Campaign” sections with a button to submit the campaign. 

After that it's just a matter of waiting since I don't believe they're currently accepting breed campaigns.