This is what I did to catch T when I first started working with her (she didn't belong to me or me to her yet:) .)
Because she was kept in a large pasture with the other horses, she did not wear a halter. So I not only had to catch her I had to put her halter on too.
Step 1. Wear very comfortable shoes, bring water (for yourself), because this will take as long as it will take.
Step 2. Besure to have a treat with you (T loves carrots).
Step 3. Locate the horse in the pasture (this can take some time depending on how large the pasture is). Once you see the horse this is important...use a signal, mine was to clap 3 times, then call her name.
Her head should come up, just out of surprise if nothing else. Stay put.
Her head will go down again, move a few more steps, don't rush this and clap 3 times , call her name again. Wait.
Continue this step, the first few times it will take a while,but eventually you will get close enough, and don't be tempted to try and grab her or put the halter on yet. Once close enough to actually be within touching distance, call her name again and offer a treat when she looks at you.
She will either move forward to take the treat or away. If she moves forward, give the treat and move away a few steps and call her again.
You want her to come to you, and eventually when she hears the clap and the call she will come to you willingly. The first few times, do not halter and lead out. Tempting, but you don't want her to associate coming to you with anything scary or unpleasant (T always made the connection of being caught with having to work).
Some have called this moment when the horse starts to follow you as the join up or hooking on. Whatever, you want her to look to you as a good place to be, treats , gentle stroking and talking.
If instead of moving to you she moves away, this is where the comfortable shoes and water come in handy. You will simply continue to move closer and stopping while clapping and calling. Try not to move head on to her but more at an angle with her shoulder. Think of the field as a huge round pen and when she turns and moves away from you, you will move to head her off and possibly turn her again. Eventually she will tire of this constant moving and will decide to look at you (this annoying person who is not getting tired and frustrated ) and will become curious and move to you.
ONce I did this with T, it only took a few times and then upon hearing the 3 claps (you can whistle or ring a bell which is what I use now) and my voice calling her she would come galloping up from the field and to the gate where I was standing.
It was easier teaching Walka this because I started working with him shortly after he was born. If he spots me, he immediately comes to me now, T will but only after the bell rings 3 times.
Be patient and consistent and when you find you're getting frustrated, take a break. As you know your horse will sense these emotions and it will affect the outcome.
Sorry this is so long and I hope I didn't forget anything. Good luck!