It is important to set one general (Default) tactic, but it can be useful to have different tactics for different situations. It can also be a good idea to have a tactic where you use your extra players, so they do not loose to much of their mood.
To create a tactic go to Squad > Senior Squad or Youth Squad > Tactics and press one of your tactics, that you wish to change or create.
At the start of a new season, when the system creates the forthcoming matches for the week, it will also set your team to play using your default tactic. If you wish to use a different tactic you must make the change yourself.
When inside a tactic you can choose which players should be at certain places. It is important that players are placed where they belong. This can seen by the green plus fields. If you place a player on an orange or red field, the player will suffer a reduction in his attributes when playing a match. This can be done by dragging and dropping a player to the field in the middle of the screen.
It can also be a good idea to set Adjustments.
This will help you keep control when certain things happen in a match, for example a player gets injured, to change the tempo of a match, or to make tactical substitutions.
You can read more about adjustments on the right side links.