Well, the first step in all of that process is actually determining whether or not you have a breeding-prospect dog

Determining meaning being able to prove it, not just think it (because everyone thinks it of their own dog, and 95% of us are wrong LOL

).
Being suitable for breeding involves two main aspects - the first is genetic health (animals that carry genetic defects should not ever be bred from), and the second involves determining whether or not the animal is a correct enough representative (in construction & temperament) of their breed, compared to the breed standard, to be used for producing the next generation. These are things that need to be tested, prior to even considering breeding (and it should go without saying that one should not consider breeding an animal that passed all these tests with another that hadn't).
So - the first steps in determining whether the dog can be considered for breeding should be the health aspects. If the dog turns out to have dysplastic hips or cardiomyopathy, for example, then there's no point in going any further - he should never ever be used to breed. So, you have to have him tested to be sure that he's genetically healthy enough to breed. The things that boxers should be tested for are arrhythic right ventricular cardiomyopathy, aortic/sub-aortic stenosis, hip dysplasia (x-rays) and hypothyroidism. Most of these can only be done with any surity after the dog is 24 months old, of course - so no boxer below that age can be considered for breeding.
At the same time, you need to establish that the dog is correct enough compared to the breed standard to be used for breeding (as this gives the best chance of producing similarly correct offspring - it should be obvious that a dog with moderate or worse faults in construction is a poor choice for breeding). This is generally established via the show ring: dog showing exists purely for the purpose of comparing individual animals to the "blueprint" for the breed (that is, the breed standard) and having an
independent expert (the judge) assess how closely they conform to that standard. Those who conform closely (in construction and temperament) will eventually make their championships, or close to it. These are the dogs who can be considered as breeding prospects, subject to satisfactory health test results.
If your dog hasn't yet gone through the above and passed both processes satisfactorily, then you cannot responsibly consider breeding him until he does. If you're not prepared to do all that (its expensive and time consuming), then you should not ever breed your dog.
Even putting aside the responsibility factor (which is enormously important - the world has no need of more irresponsibly bred dogs that can end costing the unsuspecting new owners thousands in corrective vet bills), you wouldn't find too many people interested in breeding their health-screened and champion animals with anything less than an equal
If you've
already done all of the above, then the best place to start looking for suitable animals to breed with is probably with his own breeder, and others you can meet in the show ring and via boxer breed clubs. Choosing the correct pairing isn't just a matter of putting two animals of the same breed together and letting them get on with it

Rather, the various strong and weak points of both dog and bitch need to be assessed and the most complimentary pairing chosen on that basis.