What are the best conditions to invade sinners?
When I started playing Dark Souls 2 I joined the Way of the Blue so I could get some help if I was invaded, and also because the ring was a nice HP boost. I got to SL:~60 and a soul memory of about half a million and wasn't invaded once. That's fine, but I do want some PvP in my game, so I joined the Blue Sentinals.
After I joined I put on the ring and ran around killing stuff for a while in human form. Imagine my boredom when, after an hour, I wasn't summoned to assist anyone! I figured, well that's fine, I can just invade sinners to get my covenant points! I then proceded to run around everywhere I could think of using my cracked blue orbs. There were a ton of areas that I couldn't use them at all, and every time I could, not a single match was found. I did this for about 3 more hours to no avail.
What are the best conditions for invading sinners? Do I need to raise my SL and SM more? Are there PvP hotzones (areas where invading is easier)? Do I need to be in the same area as someone being invaded to be able to help them?
Generally, you will be more likely to find people to invade as a blue phantom near the end of the game and with a high soul memory. The reason is this:
You can only invade players who have the status of a sinner. To get this, they have to have accumulated at least 10 sin points. Killing a player by invading him with the Cracked Red Eye Orb and killing an NPC both increase a player's sin count by 1. Being killed by a Blue Sentinel decreases the count by 1. (Source)
Red eye orbs can be found as treasure, but not in a great number. The only way to have a reliable endless source of these orbs is joining the Brotherhood of Blood and winning them in arena duels there. But to do this, players first have to find this covenant, which is in an optional area in Huntsman's Copse. Unlike Heide's Tower of Flame, where you can join the Blue Sentinels, this area is not available from the start, and since the order in which you progress the game is not linear, it is possible to find this covenant late into the game or not at all.
Also, the amount of NPCs in the game is low and since there is mostly no real benefit in killing NPCs (and you need the services of many), there are not many people doing that.
To add to that, you have to be within the soul memory range of the person you want to invade. Soul level does not have an influence on match-making. According to the fextralife wiki, you have to be within 25% of the other player's soul memory, but there is no proof for this value at the time. It is sure, however, that the range is a percentage, so it increases with the total amount of the memory. As soon as you reach a soul memory of 15 million (which normally takes several playthroughs or a lot of PVP), you have reached the "top tier" and can connect with anyone above 15 million souls (Source).
To sum up, while red phantom invaders can invade anyone within their soul memory range, the victims of Blue Sentinels have to meet the sinner criteria as well, which is not easily obtained, which is why it is harder for them to find other players. Generally, you will have a much easier time finding targets towards the end of the game, because the chance of players having sinned is much higher.
The reason why the Guardian's Seal has an even less chance of connecting is that the requirements for that are very limiting. To get summoned, another player has to be within your memory range and a member of the Way of Blue while being invaded by a player with a red eye orb. Since you almost never get invaded at low levels for the above reasons, players generally don't stay in this covenant for long. And when they progressed further into the game where they actually could be invaded, they'll have found the much more rewarding other covenants, so don't count on ever getting summoned with the ring. I don't know if you have to be within the same area to be summoned, because it never happened to me.
My personal experience is that, at a soul memory of about 2 million, I managed to invade frequently in Drangleic Castle. Here are some general tips for blue invasions:
Try invading in the furthest area you have progressed in, because the chance of others being within your soul memory range is the highest there.
Don't give up when the message "no world to invade" appears after using a cracked eye orb. This happens frequently, and you should just assign the orbs to a quick slot and try again right after the message. In my experience in Drangleic castle mentioned above, I managed to invade about one in every 15 orb uses.
Don't try to invade as a blue phantom before you've reached the Brotherhood of Blood area yourself, because it's likely other players in your range won't have sinned.
If you don't manage to invade at all, just keep playing. The higher your soul memory, the higher is your victim's soul memory, and so is the chance he has sinned.
Being summoned with the Guardian's Seal happens close to never, so don't expect to get any summons there.
If you haven't progressed very far in the game, but want to play PVP, your chances are far better in the other covenants. In the Bell Keeper covenant, I alerady got summoned frequently with a soul memory lower than 500k. Sometimes I couldn't even get the summoning ring off fast enough before the next one. PVP chances are also good with the Rat King covenant in the Grave of Saints. Another advantage of them is that you get valuable Pharros Lockstones and Titanite Chunks as rewards, while the Blue Sentinels get nothing but the satisfaction of punishing the guilty. With your soul level and memory, you should find good fights in both covenants.
Of course, if you don't mind becoming a sinner yourself, you could also join the Brotherhood of Blood. They always find victims, since they can invade anyone in any place.
Regarding your question about PVP hotzones: There is a hotzone in the Iron Keep, on the bridge at the first bonfire, where I always found red summon signs and dragon eyes with a memory of around 1.5 million and higher. Most players might have a higher soul memory than yours, so you should try that place later.
From what I've heard, sinners can be found frequently on NG+, because of the high soul memory range and the high chance of having sinned enough sometime during the first cycle.