Well don't start in the wrong direction...here's some pointers:
-More than 8 players in a room -> Group players. Say you're playing a 16 player garden TDM in exp2. Most of them are going to be low ping and a few will lag, or vice versa. Memorize the minority (less amount) and just lead the majority ping. What will happen in-game is that you will shoot once or twice and then realize who you're shooting at and adjust accordingly if he or she is one of the names you memorized.
-Youtube vids on lead won't help you at all. Learn by playing and experimenting! Don't memorize distances or marks or anything. Feel the time you have to lead (not any physical numbers) and you'll get it in no time. Experience will help a ton with this. Eventually you will be able to look at an opponent with say 80 ping and point to a bunch of different spots he will go. After that, it's just prediction, which is a skill as well as guessing.
-Play smart. If your opponent is low he will run for cover, he may dash away from it once or twice or just stand there to make u waste a shot. Use ideas like this to help you with predictions.
-As hinted in the first point, if you're playing a deathmatch or something..usually I don't bother learning pings at all. Why? Well, scores don't matter, and it's too much work. It's not a good idea to do so. If you can group the players, then that works perfectly. If the game has a bunch of different pings, just go with the flow. As you're playing, learn the pings of any players giving you a hard time and push back now that you know their ping (this mostly comes after learning how to lead different pings). TDMs of 12 and more are usually difficult for this reason, especially with numerous enemies running around the map. You'll find what works best for you. Worst case scenario = you forget someone's ping, try to lure him into a tight space and/or sword him quickly and you can judge his ping based on how long it takes him to flinch.
-As far as learning to lead goes.....it's a bit complicated. 1v1s are your best bet (not duels in hall). Or entering rooms where everyone is mostly one ping is really beneficial. You'll get the idea. This kind of relates to how a lagger can kill you without leading. Lead isn't just moving your crosshair away from your enemy...it's knowing where they will be. If they don't lead at all, it just means you were in the spot they shot some time ago.
I hope I helped a bit. Experience helps the most...also, it helps a ton to find a comfortable way to aim. I wouldn't try forcing anything you're not comfortable with, but I don't look at my crosshair ever. It feels weird to do that now. I look at where I want to shoot, and I'm used to my mouse's sensitivity to the point where I can just aim where I'm looking, or extremely close to it. This also gives you a bit more awareness and help with movement/map control as you aren't focusing on something on your screen but the bigger picture and your enemies. If you would like to try that, you can give it a shot. It took me about a month ish to fully grasp how to aim that way, but it was totally worth it. Noticed better aim right away.
Don't forget, at the end of the day the game is all about having fun !