Of course these are just tips, you take your safety and possibility of legal recourse into your own hands. I do not either recommend or discourage fence-hopping either way. That said, here is some actual advice.
1. Be aware of the area. Are there any houses? Is there a lot of covering vegetation, a place you can have some privacy? What is the level of activity on a weeknight? Is there an escape route or several handy, in case someone comes round? A separate paddock you can take your mare to keep the other horses from bothering you? Answer these questions for yourself first.
2. Build a rapport with your intended lover. Can you modify your routine in a way that would have you idly walking past the area? Every horse has a break-in period of varying lengths of time/amount of interaction with unfamiliar people. If you happen upon the horse's owner, try to go to work for them, tell them you love horses and are in need of some extra money. Some might have a few small tasks they would be happy to put you to around the paddocks and this will allow you plenty of time to get to know the animals on the up and up. Feed them little treats such as diced apple and beetroot. If you go that route, have a little pride in your work and do a good job. Maybe you are asked to come back for more work. And like that you're in!
3. Do some prep work before you go in for the deed. Bring treats, such as a couple cans of oats, diced apple, sugar cubes, mix it all together and bring it with you. If your intended wears a bridle pretty much all the time, a length of stout rope with a karabiner at either end to keep them from wandering off will serve. But be cautious! If they can't run then they will fight! More on that in a bit. Don't bring a car. Too limited. Too identifiable Restrict yourself to what you can bring in a small backpack and ride a bike. There aren't all lot of places a bike can't go and they're easy enough to hide, too. A change of clothes if you suspect you've been spotted. By the way, addendum to #2, look for cameras. They are usually under a roof or awning if there are any at all. If they're there, or if the owner has a sign from a home security company in their yard, just say NO.
4. Alright so you're packed and ready for your grand adventure, let's talk vigilance and personal safety. You know that feeling before sex where you're hyper focused and aware, almost as if there's an electrical charge about you and you're giddy with energy? This can be good and bad. Be aware to the surrounds at the moment, look for any sign of trouble. Better to abandon your plans than have to explain this to a judge. (I have abandoned many such encounters, and consequently, have never had to explain them to a judge.) If the coast is clear, here are some steps to take before you begin. First, walk on hard ground if you can, ground that doesn't leave too much of a track. Find a bucket for your intended's treat (Do NOT feed a horse too much oats-this is strictly a once every two weeks or so deal) or attach your rope to the bridle and find a good post or something to secure to. Remember where you got the bucket and put it, and the item you will be standing on, back exactly as you found it when you are done.
During the deed, keep a weather eye on things about you, and look for signs of frustration from your horse. They're not subtle, just quick. Start by stroking the horse's rump.if this is going over well, work inward. Horses like foreplay too. Touch it's anus or vagina gently. If they begin to pull away, you may have a recalcitrant horse and should not attempt. If they seem comfortable, slowly begin to insert a finger. Remember to stand off to the side as you do, and not directly behind. This is the point where a horse will tell you very directly that this is not going to happen. If the tail curls upward, keep building to the moment. If their rump dips and haunches bunch up, back away now. If they use their tail to seal off access to their anus or vagina, do not try to force the issue. This horse clearly doesn't want what you're selling. If, however, you get the tail lift (or even the lift and curl!) this is a sign that the horse either wants or expects to be mounted, stroke it for a few minutes before making your attempt. Again, keep an eye out for signs of frustration. Even small horses and ponies are still massive animals and are well equal to the task of fucking you up royal if you piss them off. Dipping hindquarters, tensing muscles, snorting and whinnying, attempting to get away from you, a horse will tell you everything you need to know about their temperament if you pay attention. NEVER attempt to remount a frustrated horse. Try to finish before the horse becomes frustrated or finishes it's treat. Generally not a problem.
Remember to leave everything as you found it. If it doesn't look the way the owner remembered on the previous day, they might be inclined to pay more attention.
TL;DR No, go back and read.
Again I am only offering advice here, I am not encouraging or discouraging you hop a fence and fuck a horse, I assume if your choice is made then there's very little said here that will deter you anyway. Just try to be safe and respect the animal.
Source: personal experience and general horse-sense.