Where can I advertise sire dog. I have a six year old miniature schnauzer male who is KC registered. Have no idea were to start?

Good to read this first https://forum.champdogs.co.uk/topic_show.pl?tid=55104

Generally Breeders worthy of the name will want to see a dog (so means showing or where appropriate for the breed, working your male), to assess his qualities and decide if he would suit their bitches.

They will also expect him to have had the breed specific Hereditary Health testing, with suitable results.

In my breed where numbers are low, even some champions have never sired litters due to no suitasble bitches available to them, or they just were not what the breeders needed for their girls.

By belonging to the breed community, and ahving yoru dogs seen, and tested will give the opportuity for breeders to assess his potential.

Remember you would also be to some extent ethically responsible for any puppies he sires.

he maybe KC reg but does he have endorsements on his pedigree which means anything he sires can't be registered with the KC. Also all the health tests need doing for the breed as well.
You will need to think carefully as some dogs have a massive change in their behaviour after they have been with a bitch.
Also are you prepared to be jointly responsible for any puppies he sires along with the breeder? Good stud dog owners are very vested and want to be part of the whole process and won't allow their male to be used on bitches with endorsements, non health tested and will know the bitch owners usually via shows etc and in the world of that breed.

He's 6 years old and he's presumably never been used?   Hum.   Normally somebody with a decent male will start by showing him as a puppy, and if he's successful, he will attract interest from other owners with suitably good bitches (although most with knowledge would tend to use the sire of a new dog if he's throwing good stock).   If he is that good, I'd have expected his breeder to have shown some interested in using him?

Again most people with a young successful dog will then use him themselves on one of their own bitches, one that the breeder hopes will match with the young dog.   If that mating produces good quality puppies, they may, note may, offer the dog at public stud (usually to approved bitches only).   And that means you have to have enough knowledge of the bitch concerned and her pedigree, to know whether she'd be ok mated to your dog.    The owner of the stud is 50% responsible for the puppies he may have sired, in terms of what they have to offer their breed.    I had two at public stud but turned away more enquiries than I accepted.

And then there's testing - it's not enough to have a vet certified 'healthy animal'.   Or to be KC registered!!

Finally, as already said, once a male is allowed to mate with a bitch, he will be looking for others, who probably won't be there!   At 6 years old now, your boy will be way better staying as a much loved pet.   Honestly.

I tried to use a 5yo dog which had never been used, last year. He couldn't breed and showed no interest in mounting even when the bitch was standing and practically shoving her rear in his face.

Good luck, I'd say. We gave up and used an experienced stud on her next season and had a litter...