I started playing FM when FM13 came out. Back them I had no idea how the sliders worked and downloaded tactics which I slightly tweaked. Same on FM14, but the tactic system got drastically changed and I started to get an understanding of the game.
On FM15, I was determined to create my own tactics with two exceptions. I saw somewhere on this site that closing down and tight marking all wide men is effective so I added that to all my tactics. Also, I copied set piece instructions from a few places, but nothing overpowered in them, just a few set piece goals here and there which I honestly think is fine.
I started with Newcastle and what I had in my mind was a 4-3-3 with a standard keeper, two fullbacks on support, two centebacks on defend, anchorman, box to box, roaming playmaker, two wingers on attack and an advanced forward.
I got 4th first season and after that I have won the league every year.
My team intructions are quite standard for possession based football. Shorter passing, retain possession, work ball into box, low crosses, exploit the flanks, look for overlap, close down more, use tighter marking, get stuck in, prevent short GK distribution and lower tempo. My centerbacks are set to close down less and mark tighter, fullbacks with loads of attacking instructions, anchor man set to mark tighter, my box to box is set to shorter passing and mark tighter, roaming playmaker set close down more, get stuck in and mark tighter, wingers set to roam from position and shoot less. Shoot less is also on for my fullbacks and the central midfielders.
After becoming a top team, I changed the anchor man to a deep lying playmaker set to defend with the instructions mark tighter and close down less. I also changed my left winger to inside forward after I bought Depay and my roaming playmaker to run wide with ball which makes him run out on the left a lot.
I have later created another 4-3-3 system, but with a defensive midfielder instead of the deep lying playmaker and a central midfielder on support instead of a box to box to fit certain players in my team who doesn't fit the box to box role.
My 3rd tactic is a 4-2-3-1 with a ball winning midfielder on defend and a deep lying playmaker on supporp behind an attacking midfielder on attack. The 4-2-3-1 hasn't been tested much against top teams and I still work on it making sure it's a reliable option for when I need to attack more or break down buses.
The only difference between the three tactics is some of the player roles and the formation. Anything else is the same, but I do change my instructions before certain games and I always make chances in game depending on the situation.