Figure 1: Needle insertion for superficial cervical plexus block. The needle is inserted behind the posterior border of the sternocleidomastoid muscle.
Indications: carotid endarterectomy, superficial neck surgery
Landmarks: mastoid process, sternocleidomastoid muscle, C6 transverse process
Equipment, superficial: 1½-in, 25-gauge needle
Equipment, deep: 2-in, 22-gauge, short bevel needle connected to a syringe via a flexible tubing
Local anesthetic: 15-20 mL
Cervical plexus block can be performed using two different methods. One is a deep cervical plexus block, which is essentially a paravertebral block of the C2-4 spinal nerves (roots) as they emerge from the foramina of their respective vertebrae. The other method is a superficial cervical plexus block, which is a subcutaneous blockade of the distinct nerves of the anterolateral neck. The most common clinical uses for this block are carotid endarterectomy and excision of cervical lymph nodes. The cervical plexus is anesthetized also when a large volume of local anesthetic is used for an interscalene brachial plexus block. This is because local anesthetic invariably escapes the interscalane groove and layers out underneath the deep cervical fascia where the branches of the cervical plexus are located.
The sensory distribution for the deep and superficial blocks is similar for neck surgery, so there is a trend toward favoring the superficial approach. This is because of the potentially greater risk for complications associated with the deep block, such as vertebral artery puncture, systemic toxicity, nerve root injury, and neuraxial spread of local anesthetic.
source for further details of procedure here