Practice Get a larger ring, say, a shower or curtain ring from a hardware store, but large enough diameter. It will be easier to get your testicles through this than the long back of the birdlocked. You could also practice with cable ties of even rope, but anything rigid and circular will be more the right feel. Now, pull the scrotum skin through the ring. Aim the bottom of one of your testicles at the ring and pull scrotum skin around it to get it pointing at the center hole of the ring. Push the top of the testicle gently to get it to pop through the ring. Now, slide your testicle which is through the ring to the side and hold it there. Pull skin from the other side of the scrotum to roughly aim the second testicle through the ring. Push the top again, and poof... you are through. Adjust the ring up so the scrotum is comfortably through and center. If you can do this without lube, which will be messy and cause you to lose your grip, do it. You will probably need lube to get into the birdlocked however, as it grips the skin a bit more.
I have one (not mini, but 45 mm), I also have two testicles, and I managed to put it on (several times). Patience and perseverance (and a soft penis!) are the keys. If you've never put on a cockring before, you might need a few attempts before you succeed. (You might also want to play with things such as neoprene cock rings, which are great and much easier to put on.) As described above, first pull some skin through the ring (don't forget to do that with the cage rotated down, as instructed on the web site, it's easier that way), then pull more (be careful not to pinch the skin, short nails are best), and even more, as much as you can. Since the ring has a lot of grip, while you pull the skin, pass a finger between the skin and the ring, maybe apply one very small drop of lube. When you feel you've pulled as much as you can, apply pressure on your testicle, slowly but firmly and steadily. At some point, the testicle will pop through, which is surprising the first time, but not painful. And maybe that will be enough for a first session. Make a break, keeping one testicle through. Get used to it. Then, with one testicle through, pull even more skin, as much as you can, and put the second testicle through. It won't be quite easy, but it's quite doable. After that, you'll really want to take a break, think about something else, and most importantly, get your penis soft in case it isn't. Putting the penis in the ring is rather easy when it's soft, just push it under the ring, pull the ring, put it a bit more, use a small drop of lube etc. Eventually your penis will get to the other side. Pull it as much as you can, so that the ring is as high against your abdomen as possible. If your activities have stiffened your penis (quite possible), take a break. (It is also possible you notice that 45 mm is too small for you. If your penis stays impossibly erect because of the pressure of the ring, maybe you need the 50 mm one?) Then turn the cage up, by turning the ring, not holding the cage itself (you risk tearing the cage else). Use of a bit of lube will help you greatly! Now that the cage is properly aligned above the penis, all that remains is pushing the penis inside, with lube. Be careful not to pinch the skin. Rather easy when the penis is soft. Once done and settled, wash with water in case you have put too much lube. The final lockdown with the strap and the lock is the easy final step.
The birdlocked is a very difficult device to put on because of the width of the base ring. If you've never put a cock ring over both testicles and penis before, you will have extreme difficulty figuring out how to do it, especially with this device. Suggest you try putting on multiple rings of the same size first, this will give the proper feeling. You need to be able to clasp your testicles in one hand, wrap the thumb around and pull them away from the body about 1". If you can't do that, your scrotum is too tight and needs to be stretched. You also need to be able to separate the testicles by about the long length of one testicle. Heat helps. Cold will make this very difficult. If you can do that, you can put on a birdlocked 45mm unless your testicles are over 40mm on the smallest axis. My testicles measure 2" the long way and about 1-4/16" across. I can fit both testicles and my penis through a 1-7/16" (36mm) inner diameter ring. That's borderline strangulation size when erect, but it's absolutely no issue when flaccid, so I need a clearance of only 3/16", or about 5mm over the diameter of my testicle along the smallest side. That's a damn tight result and comes from practice. By comparison, the birdlocked 45mm is over 1-12/16". Wearing it is a walk in the park for me, but putting it on is still a bitch because the damn collar is so wide. Practice - and measure. Could really be the 45mm is too small for you, but then, those are some huge balls.
The BON4 is a similar, competing device. The box contains several rings sizes. It looks much easier to put on. However, the cage is slightly larger than a regular Birdlocked, so it might be too big for you. See this thread for more information, including web site link and a nice video comparison: http://www.chastitymansion.com/forum/showthread.php?4177-Bon4
If this helps, this is the way I managed to put the ring from my Bon4 on, I have quite a tight ball sack and it was difficult for me to start with, here is how I did it though. First off, make yourself cum, it really is much easier to lock up when you are not in the mood for being locked up. What I usually do is as follows, if I'm not locked up, I send a text message to my GF saying that I'm going to lock up for her, then I will send her a pic, this way she waits for the pic, so I have to lock up. She has the keys to the lock which she leaves open when she leaves my house, so that's not an issue. Take a shower, shave your balls if that's what you're into (I find it easier to put it on if I've shaved). Get some shower gel and wash as you normally would, make sure your cock is soft then reach through the ring and grab some of the skin of the ball sack, pull as much of it through as you possibly can. Push the testicle through from the other side of the ring. With one testicle through, pull as much skin through the the ring as you can, then push the other testicle through the ring. It really is much easier if you are in the shower when you do this as the shower gel is a good lubricant. What you should have now is two balls through the ring, and if you're like me a cock that's trying to get hard. Let it subside for a few moment, then push the head of the penis down through the ring and push it with your index finger until it's all the way through. That works for me, hope it does for you too. If not, my GF came up with an idea that gave me an incentive to get it on, she would have been willing to use a corset lace to stretch my ball sack.. As kinky as this might of sounded, I prefer the stretchy silicone to the tightness of a corset lace. So I got it sorted. Scuba..
Make sure not to get a testicle rotated around while putting on the birdcage. This is rather painful :\