As always, make sure bdsm/kink play is safe, sane, and consensual. When using e-stim, keep it below the heart to be safe (cock, balls, feet, etc.) and test it on yourself first, on the areas that you intend to use it on with your sub, to get a feel for what each pattern and intensity level feels like. That way you can avoid boring your sub by keeping things too weak the entire time, as well as avoid really hurting him or freaking him out so he'll never try e-stim again. I realize a Dom/Master likes doing things TO his sub, but let's be responsible and at least briefly try it on ourselves first just to keep things safe. Keep in mind, once you find your ideal range, your sub's range may be somewhat different. Keep an open-mind and realize that with e-stim play, it takes time to find the ideal range of intensities and patterns that aren't a complete turn-off for your sub.

Gather metallic objects of your choice from around the house (paperclips, coins, binder clips, small padlocks, latch clips or carabiner clips from your gear stash, etc.), and scatter them and the magnets that are included with the nipple clamps around the room you'll be using for your session. Just make sure they're visible to your sub, even if it's a challenge to find some of them.

Choose an e-stim audio file from this site or a similar audio file somewhere else which ramps up over time. Choose an audio file with a reasonable time length depending on how many objects you scatter around the room (i.e. If you have 50 objects laying around the room, it wouldn't be very fair to your sub to make the e-stim almost unbearable after 2 minutes. You want to motivate him, not give him an impossible task.)  Load your audio file onto your mp3 player and ensure you have a cable that will connect your line out jack to the line in jack on your e-stim power box. Test it to be sure it activates the e-stim. (Nothing is more of a kink turn-off than having everything setup and realizing your line out/in doesn't work, and killing the hot session you had prepared, that fast.)  ;)

If you want to be an extra responible Dom, do a test run. While playing your mp3 file so you can hear it, crawl around the room to each of the objects on the floor, making sure you can get to all of them before the audio file reaches it's louder/more intense parts.

Optionally blindfold your sub while away from the playroom, if you want the session to be a surprise.  Then cuff your sub's hands in front of him.

If you opted for a humbler, have your sub get on his hands and knees so you can lock the humbler on him.

Put your e-stim attachments on/in your sub.

Using the bondage tape or some other secure method, attach your sound activated e-stim power box to your sub's arm or leg, or anywhere it won't easily fall from. Do the same to attach your mp3 player near the e-stim power box.

Connect the e-stim power box to the e-stim accessories, and connect the line out jack on your mp3 player to the line in jack on your power box. (Be careful on this part. Connecting to the wrong jack on your power box can damage some devices.)

Put the nipple clamps with magnetic weights on your sub's nipples.

Have your sub get on his hands and knees. Remove the blindfold you may have used, and explain the rules of the game to your sub while you watch his wide-eyed expression as he looks around the room.

The rules could go something like this:

"I've prepared a fun game for you, boy. I dropped a few things around the room, and I want you to pick them up for me. For some reason, you don't seem very motivated to help me, boy. Good thing I prepared a little motivation for you. In a minute, I'm going to turn on the sound activated e-stim power box that I taped to your body. It's connected to the [rubber loops around your cock/bands around your balls/plug in your ass/humbler locked on your balls]. I prepared an audio file that will feed into the e-stim box which will motivate you to work quickly and efficiently. You'll soon notice that the longer it takes you to pick everything up, the more intense the e-stim will get. You're going to use those magnetic nipple clamps to pick everything up. The e-stim box will be turned off after you've picked up all metalic objects from the floor. The slower you do that, the more intense the e-stim will get before it's turned off. However, if you move too fast, those nipple clamps will swing around more and they'll get heavier as you pick up more objects with them. It's up to you to come up with a good balance while you clean this room up."

1. Instead of using a cable to connect your mp3 line out jack to your e-stim power box line in jack, you could just have the audio file playing out loud in the center of the room. Then maybe hang some bells from your sub (or even have a few bells laying around the room for him to pick up). You could even take bells from old pet toys and use them for this purpose. Then when your sub crawls around the room, especially if he tries to move to fast, the bells will activate the e-stim too.
2. You could also require your sub to pick the objects up in a specific order that requires him to crawl around the room more. For example, one object from the left side of the room, then one object from the right side of the room, alternating back and forth. Or, have him pick up all paperclips first, then all binder clips next, etc. If you have a lot of objects, have him return to you after each type of object so you can remove that set from the nipple clamps before he continues.
3. You could also set a cage up and put some of the objects in the cage, even really close to the bars. That will make it more of a challenge for your sub to pick up the metallic objects with the magnetic nipple clamps without having them get pulled in by the cage bars.
4. The magnetic nipple clamps don't have to be used on your sub's nipples. Some subs are overly sensitive to that, or maybe you just want to switch things up a bit. Try hanging one (or both) from a rope tied around your sub's balls for a no pinch, just weight, approach.
5. If you want to keep your sub guessing or just train him to follow orders without question, you could choose to not tell him about the intensity increasing over time. Then when he feels it kick up the first time, see if he figure it out and works faster. ;)