What they said

TSC is currently selling the box of five 10" brace pins for $2.44. Website says "was $4.89" and I think that is what I paid. Maybe it is a 'not fencing season' thing. They also have 5" pins. Depends on what size posts you are using. Can't use a 5" pin if you are using a 6" post and either want to drill through into the horizontal member like I suggested earlier or want to leave 1/2" or so out as
@Latestarter said to 'anchor' the brace wire to keep it from sliding down the post. The pins don't pull the posts together, they just keep the horizontal from slipping down, same as the notch and BIG nail on the left side of
@greybeard's drawing above.
If you don't have a pin sticking out, you use fence staples as is commonly done at the bottom. That is shown in a number of videos but as I mentioned earlier, trying to hit a hole that you can't see in post with the pin sticking out of a heavy long horizontal member isn't much fun. I found it much easier to just anchor the horizontal where I want it with ratchet straps and drill through the post into the horizontal then pound in the pin whether I needed to have a brace wire there or not. Having the horizontal cranked tight during the pin pounding process makes that easier as well since there will be less "bounce". It isn't until the brace wire is in place that the H-brace is really solid but the ratchet straps keep it stiff while pounding the pins. Oh, and I misstated the drilling process before. I don't need a level for that, I can eyeball it. I use the level to drill the holes for the bolt hooks to hang the gates on.
If you are going to hang a gate on one side or the other, you need to use an H-brace. In the drawing above, it is braced for a gate or stretched fence on the left. To put a gate or fence on the right, you would need brace wire in the opposite direction. If you have both stretched fence and a gate on the H-brace, you need brace wire in both directions.
With all the @#$%^ rocks and ledge here, I did use floating braces where I could. One less post and brace pin to buy, 1 less pin hole (or notch) to make but more importantly one less 3' deep hole to dig. On the other hand, if you have easy digging ground and a power auger, you can avoid figuring out and buying/finding what you are going to use for the ground pad and cutting on both ends of the angle brace part. As noted above I set the posts for my H-braces with the horizontal member on the ground as a spacer so I didn't have to cut it to length.
You can learn a lot by watching fencing videos by RedBrand and others. But they never have rocks or heavy soil and always have helpers when necessary

You will find a lot that use "twitch sticks" to tighten the brace wire. Personally, while inline fence strainers are much more expensive than a stick, I used them. Much more control and you aren't relying on a 'stick' (could be a metal rod) twisted in the wire to hold it.