lol k.
You cant have unlimited attunment slots, at least not with this trainer. The methods I list will start in descending order, the worst online transgression to the quietest method.
1. Setting your attunement stat (ATN) to 99 with the trainer.
2. set souls and level up your character in game, ATN anywhere between 75-99. At 99, I'm fairly sure all the attunment slots will be open no matter what miracles, pyromancies, sorceries, etc. you choose to put in each slot. This is equal to the first method in terms of how easily you'll get caught, because setting souls and leveling up wont be recorded in your soul memory. The actual difference between the two of course is that your level will rise accordingly with this method (and thus the upgrade cost of other stats will rise) whereas the second method will not match your level nor soul memory.
3. acquire soul in-game, preferably soul of a hero since you'll be able to mass souls much faster, and edit its quantity high enough so that you'll have enough souls to level up your ATN stat to at least 75, and so that the soul memory will match with the upgrades you've made to your attunement stat. You said you're a single player guy so I guess it really doesn't matter, but Ill say it anyway: If you do this method, make sure you've sunken in lots of hours in this game. I dont think I have to explain why.