It's why I see a physiotherapist regularly. We're establishing graded exercise in a way that avoids the crash cycle that happens so often in ME.
In essence, the way it works is that you spend months regulating and monitoring your daily routine so that things are stable. This involves all mental and physical activity. Everything is mapped and monitored, and when crashes occur you track back to what you'd done that caused the crash. By doing this you can work out reaction times and also what activities cause the problems. My reaction time is 24 hours, which is why I'm only supposed to exercise on alternate days - it gives me an appropriate level of recovery time.
It took a surprising number of months to get things stable. Even as recently as Jan/Feb I'd be stuck in bed for whole days.
Graded exercise followed on from there. I gave Tai Chi a go, then coaxed my physio to let me try the cross trainer on a low setting. The basics of it follow that I maintain the same intensity of the exercise, but gradually build up the length of time. I increase one session per week by a minute. If there is no negative reaction, then I can take another session up by a minute and so on. It's very much a long haul thing - increase too much or increase time and intensity and a crash is likely to happen.
When I've got the 25 mins established without issues, then I'm going to stick to the length of time and slowly start to work up the intensity levels.
It's all a very careful balancing act. I wasn't certain it would work to begin with, but it's been quite life-changing in many regards.
I would suggest that you actually talk to a suitably qualified physio - exercise (if tackled wrongly) might be extremely damaging to someone with CFS. It needs to be low-impact and very, very carefully managed to allow for reaction times. It also shouldn't even be considered until stability is managed - if someone is swinging from crash to crash, then exercise is just going to exacerbate the medical issues.