Self explanatory.
You can kill either the pigs in Majula or farm the torturers in Huntman's Corpse area. Both of these drop them. I would suggest joining covenant of champions before you do this otherwise they eventually stop spawning.
Items to use for farming are: prisoner's hood + prisoner's tatters + gold serpant ring +2. To name a few. You can also pop a lucky coin before you start killing the torturers for better drop chance.

Ideally you want a decent amount to keep yourself busy for about an hour. 20-25 will do nicely. 30+ to save time between trips.

This is for windows only. No idea what you do for mac or linux so dont quote me.
You can find your save file in this directory

C:\Users\User\AppData\Roaming\DarkSoulsII\0110000104907266

Your save file will be inside.
Now you need to copy that save file to another open file somewhere else. I keep mine in a folder on the desktop.

Keep this file for later you will need it.

Pick your SL and where you want to invade. Not really hard.

Around 500k soul memory you can invade pretty much anywhere early game except forest. Generally with the way soul memory works is the good players who dont lose souls min max better than players who lose souls.

Around 1m soul memory you can invade early-mid game areas like the lord soul regions. A lot of builds can be finished around 1m to 1.5m. I generally take these invasions a little more seriously. Newer players will still not be able to kill you if you really know what you are doing.

Around 1.5m soul memory you are invading late game areas like shrine, aldia's keep, crypt, etc. Expect people to have finished builds and to be able to handle you in 1v1s

Use up all of your cracked eyes and move onto step 4.

Used up all your cracked eye orbs? No problem. Remember that extra save file you created?
You're gonna need that now.

Find the extra copy of your save file you set aside. Copy it now and go back to the save directory in your app data found in. C:\Users\User\AppData\Roaming\DarkSoulsII\0110000104907266.

Now paste the copy of the save file into the directory. Again dont just move the save over you want to copy and paste. That way you still keep the spare save so you can repeat this process as many times as you want.

Load back into the game. Notice how you have all your red eye orbs again?

Repeat step 3 and 4 again and again until you get bored.