The Springer is a great tool for exercising a dog. It clamps to your bicycle and lets you safely ride with her on a short leash extending from the bike on a spring; the spring absorbs the force if she veers, so she won't tip you over. But the instructions that came with it don't tell you a lot of things that you really should know to get the best use of it. Here are the things I've learned.
Installation: Follow the directions to see how many of the plastic inserts to put inside the clamp, and choose the highest number they say, then add two more inserts. (They'll send you extras if you need them.) If the clamp feels like it won't quite close all the way, that's perfect. They're going to compress and it'll fit fine later, without you having to disassemble and reassemble it to put more in.
Tightening: Leave the wrenches out when you're done with assembly. You're going to need to retighten all the bolts after every bikeride for the first couple of dozen trips. Then you'll still need to retighten, just less and less often.
Right Angle: During use, the Springer will tend to get tugged to a position off of a right angle, usually back, and you might be tempted to pull it farther back to avoid it interfering with your foot. Don't. It'll least interfere with your foot if it's at a perfect right angle, so keep readjusting it to that. It doesn't seem that way at first, but that's how it turns out.
Extra Parts: Yes, you need to buy a set of extra breakaways and clips. Also, bring the leash to the hardware store to buy a few yards of flagpole rope of the same size. Cut an extra 18" leash and knot it with an extra clip and breakaway, then tuck this into the fanny-pack or whatever else you're using on your trips. That way, when your dog breaks off the Springer, you'll be all set to get her back on quickly.
Separate Leash: You may be tempted to keep a second leash running to your dog so when she gets loose via breaking the breakaway or something else going wrong, she won't be able to run off and get lost. Don't do this. This completely subverts the safety features of the Springer and can lead to grievous injury. You're just going to have to be ready to find her and lure her back the old fashioned way if she gets away -- and if you can't do that, you can't use a Springer.
Clipping: Every time you get your dog to come over to the bike and get clipped to the Springer, give her a treat. You want to encourage her to like coming over to the Springer, and while "we're going for a ride!" might be a good start, it won't always work, and you also want her to be just as eager to get back onto it when it's time to go home. You'll be glad you started this right from day one.
No Tipping: After she's clipped on, do not ever step away from the bike, or even move to anywhere where you can't steady the bike instantly. If she moves even a few inches while the bike's on its kickstand, for any reason, she'll tip the bike over onto herself. She probably won't be hurt, but she'll become terrified of the bike and the Springer, and you'll have a very hard time getting her onto the Springer thereafter. Be sure you're completely ready to go before you clip her.
Steady Pace: Once you're riding, keep up a steady pace. As long as you're moving at a reasonable speed, her instinct will be "keep up with the pack," so she will run along with you and the ride will go smooth. Once you slow down to a certain threshold speed, suddenly her instincts will switch to "I have time to check out smells and sounds and still keep up with the pack", and then she'll be pulling the bike back or to the side, which slows you down more and makes the whole trip far harder, plus it's worse exercise. If you have to slow down temporarily, it's better to stop.
Don't Go Too Fast: On a steep downhill, you can probably get going so fast that it will be uncomfortable for your dog to keep up, especially if she's small or old. But she won't complain; she'll work herself to any extent necessary to keep up with the pack, even to the detriment of her own health, because that's how her instincts work. (In the wild, a dog that gets separated from her pack is in worse shape than one who wears herself out keeping up.) For a healthy young large dog, it'll be hard to go too fast even on a steep hill (sometimes when I think I'm going too fast, a moment later Socks sniffs something ahead and is suddenly going faster than I am and pulling me forward), but for smaller and older dogs it might be easy.
Electric Assist: If you have a hard time keeping up a speed that keeps your dog going forward, you might find an electric-assist bicycle to be a great solution. For me, it's because of living in a hilly area; I have no trouble keeping her engaged on the flat stretches, but going uphill I slow enough that she's now pulling back, and suddenly a short stretch of road becomes a nightmare. Prices vary widely, though. I've seen electric-assist bikes, or add-on kits, for over a thousand dollars, but I bought my bike new for just over $200. Note that you might find yourself using the electric assist on a low setting even on stretches of road you could easily handle, because the weight of the battery and engine, and the slight drag of the transmission, will make the electric bike a little harder to pedal than a regular one, but the motor can easily make up the difference.