You open a portal into the Shade, and draw out the target's shadow with its blood, which then consumes the target's life force and gains intelligence and corporeality. To do so you must prepare a summoning circle drawn with onyx sand, and the incantation written around in silver dust while saying the words aloud. This process takes one hour, after which the circle is ready to be activated until the next dawn. Once the target is placed upon the circle, the caster can draw the blood and complete the spell as an Action.
When the creature appears, it is under no compulsion to behave in any particular way. You can ask the creature to perform a service in exchange for payment, but it isn't obliged to do so. The requested task could range from simple (fly us across the chasm, or help us fight a battle) to complex (spy on our enemies, or protect us during our foray into the dungeon). You must be able to communicate with the creature to bargain for its services.
Exerting control over the creature has varying requirements according to the class of the caster. The total CR of the controlled creatures cannot exceed the player's s Constitution modifier + half of their player level. A Sorcerer of the Shadow Magic bloodline can also add their Charisma modifier to this total.
Any additional creatures created that exceed this limit are not under the control of the caster and can act according to their desires. Often this means they will be hostile the any creatures not from the Shade, including their creator. The exception to this is when the creator is Sorcerer of the Shadow Magic bloodline, in which case they will not be hostile without reason.
A creature enlisted to join your group counts as a member of it, receiving a full share of experience points awarded.
To create a Shadow Mastiff, the sacrifice needs to be a dog or wolf. A humanoid must be sacrificed in order to create a Shadow Demon. Additional creatures can be sacrificed this way by adding 2 points to the challenge rating, increasing their total Hit Dice by 3 and giving them the Shadow Blend, Sunlight Weakness, and Ethereal Awareness as they are found in the Shadow Mastiff stat block in Volo's Guide to Monsters.
* - (100 gp worth of onyx sand, 50 gp worth of silver dust, 1 drop of targets blood, 4 bags of black tea.)