I actually wanted to write a tutorial about this!
get yourself an achievement tracker here- being active on the forum can help you in earning some money! 200 achievement points will earn you 10,000 $. you can simply earn achievement points by joining Slack (around 40 points if I remember correctly), registering realistic horses, completing your stable site, entering your horse in randomized shows (1 point for each entry) or in combined shows (20 points for each entry).
host a little auction with rare or colourful horses. I wouldn't set the prize so high if they are foundation horses (around 2000-4000$)
enter your horses in shows. This way you will make money and get your horses pedigree points which makes them more desirable for future breeding programs
Lease horses. This means someone lends you their horses to enter them in shows. A lot of people are generous and will award you extra money for Bonus points you earn!
Hope this helped!