In Situ
Chastity

Related
Cock and Ball
Cock Locks
Slave Castration
Electro
Bliss Elevation
Bondage
Chastity Control
Trainer Positions

There are various objectives for binding, for its erotic value, to keeping the cock and balls correctly place when you are using him or part of his Chastity Training to name but a few.
You have the further options of allowing him a method of humping himself  (normally the only allowable way for a dog), you may also keep his meat in position should you wish to  keep a catheter (piss or electrical) it will lessen the chances of it coming loose once in place.

It is important for him to know that his body is now under your control, placing each part under meticulous bondage is just one way. A slave need continual bondage to keep him in the right frame of mind!
At one level he never wants to feel free, but always a captive under some sense on bondage. Part of your Total Control Training of him is that he will learn that he knows he must take it, however long it takes, that the outcome is quite inevitable.

Keeping the cock and balls correctly place can also be done with binding. Securely placing the rope around his balls, with part of the cord used to separate each ball. The remaining cord is then used to lash down his cock, both keeping his meat firm and secure.

In addition I like to bind my pups cock and balls as  to keep it nice and hard, but without the possibility of wanking off.  This is useful for keeping him on heat for some of his harder training where he maybe allowed to stroke his doggy cock but that is as far as it goes. This is often used in conjunction with fist mitts/dog paws so that the animal cannot get access to free himself.
The Balls
One method is to use a fine leather lace (1mm by 2mm in diameter). The first is knot is at the base of the cock (where the cock ring normally goes). This need not be too tight, the next then goes around just the cock with a knot at the top of the balls/base of the cock. Keeping the two laces together thread then under the lace that circles the bottom of the balls (where a gauche might go. With care pull the lace through as this will cause friction burns on the skin otherwise. Pull this tight enough to cause the balls to be separated.
Then tie a knot in the front in the front and repeat with another knot at the base of the cock and top of the balls. The lace can then be passed down between the separated balls looping around to the front where it can be tied off. This allows the slaves balls to be further separated. For short periods you might like a figure of 8  binding around the balls. But the tighter it is the less time it can stay on. the balls of some  slaves can swell quickly and cause real pain and you will need to be able to remove the binding easily if needed.

The cock
This example assumes that the slaves cock has a PA in it and that his foreskin is pulled back or that he is cut.
Thread the lace through the PA and tie a knot in the front of the cock head, so that there is a loop the encircles the crown of the cock head. Ensure that you do not trap and of the skin in the bindings. Then with the lace, zig-zag the binding down the cock with the know alternating between front and back. The spacing is about 25mm to an inch apart, then secure the ends of the lacing into the binding of the balls.

This results in an effective cock ring keeping the dogs cock nice and hard, with a nice feeling of  the shaft being gripped. But preventing the cock being wanked (in a fist) as there is no possibility of skin movement. Because it provides a nice grip when the cock is anything but soft the slave is nicely aroused. The PA ensures that the bindings do not slip off, and when the cock is very hard the PA is nicely tugged hard, as the erection increases.

The cock can be safely covered with a condom and if the slave is required to fuck another slave it will be a particularly hard fuck leaving the other slave sore.

Removal
This has to be done with some care as the lace can 'stick' to the skin a little. Further there might be some swelling, so always keep an eye on the slave to ensure that he is find. If it is done too tight 'blisters' may form (which will go down in time). This is where there has been some pinching or abrasion.  If there is to be movement you might want to use some lube.
If the cock is free between the animals legs it can be left dry.