A rubber bit port (also called a juba tongue port) is a $2 piece of molded rubber that attaches to your pony's bit. Its purpose is to hold the pony's tongue down and prevent him getting his tongue over the bit. Below is step by step guide with photos illustrating how to attach it to a simple rubber bit (I use a straight mouth rubber snaffle in this tutorial).
Before we start here is the overall idea of what we are going to do:
Now that you have a general idea of how the juba port will attach to the bit, let's get started:
Align the Bit and Bit Port Start by placing the bit on a level surface. If possible, try to stand the bit on its rings as shown below. Position the bit port with the long portion (the part that will sit on top of the pony's tongue) facing away from the bit.
Slide the Bit Port Against the Mouthpiece of the Bit Pull the bit port against the bit. The side of the bit port you aligned in the previous step is grooved. You will slide the mouthpiece of the bit into this groove. Thus, the side of the bit port with the two holes (bottom) should now be under the mouthpiece. The other side of the bit port (top) should be on top of the bit as shown below.
Confirm Positioning of the Bit Port with the Bit The photo below shows a close-up of the bit port against the bit mouthpiece. Notice that the side of the bit port with holes is under the bit.
Wrap the Bit Port "Arms" Around the Bit Now press the mouthpiece of the bit firmly into the groove of the rubber bit port. You will have to press it firmly enough that the little "arms" of the bit port wrap around the mouthpiece of the bit as shown in the photo below. The arms should be on the top of the bit, while the side of the bit port with the two holes should be on the bottom of the bit.
Pull the Tongue of the Bit Port Over the Bit and Through the Hole You want to pull the tongue (long portion) of the bit port over top of the bit and through the first hole of the bit port (which should be under the bit). This part is a little tricky because it is a tight fit. The way I do it is I hold the bit port against the bit with my left hand, and hold the hole of the bit port with my right hand. I then use my left thumb to push the tongue portion of over the bit and into the hole.
Pull the Tongue Portion All the Way Through Once you have threaded the tongue portion partway through the opposite hole, just pull the rest of the tongue portion through - this should be easy. After you have pulled it all the way through, there is a little piece of rubber (the far end of the bottom portion of the bit port) that should snap into a groove at the end of the tongue portion closest to the bit mouthpiece. Once that snaps in place, you are done.
Done! You are now done. Flip the bit over to see your handiwork. When putting it into your ponies mouth, you will want the flat portion facing up (towards the roof of your pony's mouth). The other side has a slight curve downwards to help hold the pony's tongue down (in the photo above you can see this). The photo below shows the bit port with the top up (i.e. the side of the bit port you see in the photo below is the side you want facing the roof of your pony's mouth).
Dover Saddlery sells rubber bit ports. You will probably pay more for shipping than the juba tongue port itself, so you may be better off buying from a local tack shop.
You should hopefully now be able to attach a juba tongue port to your pony's bit. The attachment procedure is most difficult for thicker bits (such as the rubber straight mouth snaffle used in this example). If you have a bit with a thinner mouthpiece (e.g. some driving bits), you should have no difficulties.