Hello Fr. Mike, I am sorry you have gotton an invitation to this club we have.
If you do some searching on the internet you will find pictures and drawings of the neck dissection. I would recommend you NOT go looking until after you have one.

The look of it is worse than it really is. The procedure consists of an incision from ear to collar bone and from the chin, under the jaw to the other incision kind of a T shape. From there the surgeon has access to the lymph nodes and tissues that populate the neck.
The standard dissection also includes removing the Sternocleidomastoid muscle, a nerve just under that, and the jugular vein. The nerve mentioned operates some of the shoulder muscles on the affected side. These muscles will no longer function and over time the others will take up the movement of the shoulder.
The dissection has variations where the muscles or nerves are not removed. The purpose of the surgery is to get at and remove as many of the lymph nodes and pathways as possible because these are the most probable paths a cancer might take if it has spread outside the orgional tumor. It is a procedure that has been used for some 100 years.
The surgery takes maybe 3 to 5 hours and recovery time (hospital stay) 5 to 7 days. I had a tonsillectomy at the same time and my throat hurt much worse than the neck dissection. (part of the reason is that a good amount of the neck will be numb for quite a while) I did not have tongue surgery so I cannot comment on that. I wish you well and have included you in my prayers.