sorry, was having trouble .. trying again
I thought from the title of the post that you meant you had clipped him for the first time
Some might find the term "shag" a little offensive, or at least unprofessional, so just a friendly suggestion that you do use different terms.
But anyway ... you're saying that he covered or bred a mare for the first time and you have some questions...
First, it is true as someone said that a young stallion often is not successful in getting mares pregnant. Some of this will be due to inexperience, and some due to lower sperm count. We successfully bred mares when my stallion was 2, but his lack of experience meant that they did not get bred during their first cycle.
okay..
If you pasture breed, then you can just let nature take it's course and record the days you see her in cycle. When both are ready and mature enougn, it will happen. If both are mature enough and the mare doesn't catch, then it's time to get a vet involved. Might not happen for a 2 yr old stallion.
I personally do not pasture breed. To hand breed, I tie the mare, lead the stallion up to her, let him tease a bit, and see if she is FULLY receptive. Don't RAPE your mare by forcing her to be bred before she is receptive! An older experienced stallion probably won't cover her anyway when she is not receptive, but some will mount anything that doesn't move. If she is not ready, she won't catch anyway (most mares are in heat for as long as 5 days before they ovulate) and you or the stallion could get kicked not to mention the potential for damage to your mare.
Since you are inexperienced, get some help and have someone else hold the mare while you take care of the stallion. When and only when he is fully erect do I allow him to mount. With an inexperienced stallion you may need to help him enter. If he is a lot smaller than the mare, you may have to help him stay on or position the mare so she is standing downhill from him. He will visibly relax when he is finished. Mine literally almost falls asleep on top of the mare and I have to pull him off!
We breed every other day starting on day 3 or day 4. No need to breed more often as sperm lives up to 48 hours. Many of my mares whose cycles I know really well, only are covered once and we have almost 100% conception rate on them.
I agree with everyone who said you should really get advice from a local breeder. There are some things that you just don't learn by reading.