You attempt to interrupt a creature that you can see in the process of casting a spell. Invisible creatures or spells cast without verbal or somatic components cannot be targeted. 

You are able to determine the level of the spell and the nature of the spell before you decide to cast Counterspell. 

Creatures that use innate spellcasting abilities or items that replicate spell effects (like the Necklace of Fireballs) cannot be targeted with Counterspell. Creatures that use items to cast a spell (like the Staff of Power) can be targeted with Counterspell.

Make an ability check using your spellcasting modifier. The spellcasting modifier does not typically include a proficiency bonus, but skills such as Jack of All Trades and Improved Abjuration that specify otherwise do provide an additional bonus.

The DC equals <DC Floor> + [<Nerf Coefficient> * (the target spell's level - the Counterspell's level)].

The DC Floor can be increased to raise the difficulty of Counterspelling all spells gradually.

The Nerf Coefficient can be increased to increase the benefit of casting Counterspell at a higher level

We recommend DC Floor = 8 and a Nerf Coefficient = 4 to offset to the increase of effectiveness to Counterspell granted by knowing the spell that is being targetted.  

On a success, the creature's spell fails and has no effect. The spell slot is lost.

At Higher Levels. Casting this spell at a higher level reduces the DC required to succeed.