Haha... It's hard, isn't it!
I first pick a base I would like the foal to be - grey, black, white, palomino, red, or brown (I use plain colour terms as coat colours can be hard to understand at times). Then I choose several mares and several stallions of that colour. The reason I do this is because if you have two different base colours, the foal is more likely to be an unpredictable colour, which is fun unless you want a certain colored foal.
I also choose the mares and stallions based on quipping, depending if I want a complex foal or not. I then PM all the owners of the WMEs I have chosen - not only the WMEs down for 'equidless player breedings' have owners willing for others to breed them, but also, not every equid has all it's slots free, so it's good to have a few you can go to if the 'Perfect WME' isn't available.
But you don't have to take my advice - just two beautiful equids can make a gorgeous foal.