While there are more variations to the flat set than there are to the dirt hole, I'll try to come up with a general flat set that is generic in nature.
Again, location is the most important aspect of any set.
If the animals do not travel close enough to notice the set, it won't matter how well the set is constructed.
A flat set is nothing more than a set using an object above ground rather than a hole to apply scent and attract the animal.
The object can be a rock, piece of sod, piece of wood, a bone, piece of charred wood etc.
If one of these objects don't already exist at the good location, you can easily put one right where you'd like to construct the urine post set.
It's good to position the set where the object will stand out in contrast to the immediate area.
When you position the object to be used in the urine post set, keep in mind how you want the animal to approach the set.
Using existing backing material at the site you can position the set so that the animal must travel over the trap to reach the scented object.
The target animal will dictate how far back the trap will be from the object.
Of course foxes will require the trap to be positioned closer to the object than a coyote because of the difference in the length of their legs.
Each urine post set is different and each set has its own problems and peculiar things about it that governs how far from the post to position the trap.
The best thing to do is look at where you think your target animal will step and set the trap there.
This is one of those learning things that will require you to be observant of where a missed animal has stepped and from that you will get a feel of exactly where the trap should be placed over time.
You should bed your trap solidly just like the dirt hole set.
Usually a good gland lure or urine works good at urine post sets.
Before I'd make a urine post set with both gland lure and urine, I'd make two separate urine post set from five to thirty steps apart and use gland lure only on one and urine only on the other.
Having two sets that smell differently doubles your chances at any given location.
There are not target animals that can get in one, leaving one still operating.
Quite often fox and coyotes will travel in pairs.
Having two or more sets at a good location gives you a chance for a double catch.
Double catches are more common in the fall and winter than in the spring and summer months.
This just a basic description of a Flat/Urine post set.
There is a lot more that can be learned about the urine post sets.
You can learn more by getting the book by Charles Dobbins Variations of the Flat Set.