When and if i am symptom free, i start with stretches and progressively get to simple exercises with resistance bands for my upper body and maybe some ab and core exercises (like a plank pose), and leg lifts and squats. I did this at my home so i could rest if i became fatigued.

If i get a headache, i stop, then try again in a day or two depending on how i feel. I did this starting in my 2nd month post injury and progessively added exercises, like walking on a treadmill for 5 minutes at a time. Each day i would add 5 minutes to the treadmill as long as i wasnt dizzy or have a headache or too fatigued. Never went past 30 minutes and always tried to keep my HR under 100 (as suggested in the Buffalo clinic study mark referenced)

I chose a treadmill so i wouldnt get caught in walking outside too far and worried about having to come all the way back. I also have a small gym at my apartment complex, so i would go there to walk on the treadmill so i was close to home if i needed it

Of course, when i was caught up in my very fatigued, headache-plagued stages of recovery (like when i returned to work for 2 months), i had no energy to do anything more than simple stretches in my living room. Everyones exercise tolerance is different, so listen to your body.