Here's one example-
Although there's a lot more to stabilizing a neck than this, this is often a good start.
Place your fingers against your temple and turn your head. Neither your head nor your hand will actually move, just provide the mild resistance. On a scale of 0-10, start by providing 2/10 intensity resistance. Hold for 3 seconds. Relax. Repeat. Repeat until your neck starts to feel some fatigue and then stop. Repeat on the other side. As you continue this exercise daily, your repititions will improve before fatigue sets in.
Here is a picture to help-
These simple stabilization exercises are a great way to get a head start (-no pun intended) on having a flexible and stable neck again!