
You need an experienced person at the actual matings or to have been at matings yourself really.
When he mounts, you will know when he has hit the mark and is ejaculating as he will look as if he is trying to climb a ladder.
To achieve a tie it is often helpful at this point to not allow him to try to turn for at least a minute (as he has been pulling himself free), this is called 'slip mating' a tie is preferable as it causes a seal which helps send the swimmers along, and also the dog continues to ejaculate during the tie, mostly prostatic fluid by then, which feeds the sperm, which may be why slip matings are less likely to ge5t a bitch pregnant.
By holding him in the dog on top position, or then helping him dismount to stand side by side, or he may wish to turn, so you can steady them while he gets his leg over.
It may be an old wives tale, but it is often said slip matings often happen when the bitch isn't quite ready.
You say the stud is experienced, does he normally 'tie' or are 'slip matings' normal for him, as they quite often are with some toy and some other breeds.