Only tried voluntary work about 10 years ago, and that was only 1-2 afternoons per week... Only managed 3 weeks in a row before the crash though.
I then tried to do voluntary work again a year later, working at a drop in centre and needle exchange for people with HIV but they didn't want someone with ME helping them in case I made them ill lol.
Ever since it has been singular one off events, along the lines of homeless soup kitchens; knowing full well i can't keep doing things every week and maintain it.
Its so difficult recommending types of work to prepare for because it depends on your symptoms and severity and ability to repeat things either daily or weekly.
Working from home would always be the ideal but then you lose the social aspect if you need this.
Sitting at a desk might be helpful if the chair isn't painful and the environment (lights, noises, smells, people) aren't too assaulting to the body.
Maybe start by volunteering first, to test your stamina and social stamina. If theres any ME groups locally, see if they need any help in the office, or with newsletters or answering the phone. They're the best people to test yourself on as they understand. If it goes well, you can use this to demonstrate to employers that you're capable of certain tasks etc. and they'll give you a reference.
ME is still widely unrecognised, and this can be used to your advantage when explaining yourself to employers, especially if its mild and its not as obvious.
You could try working in a charity shop starting with an hour or two a week, volunteering to begin with.
Its so difficult because of fluctuations. You maybe be capable of doing something one month then nothing for the next 3.
For me personally, i've just done many courses over the years from college to open uni, most of them at home. So i now have a collection of qualifications for if/when I can work again.
Explaining the need to be absent... I'm not sure about. Back when i worked, its not something i ever sucessfully managed to achieve without being threatened with losing my job. I'd think flexi time is the best option but i don't know how easy it is to get this these days.
There is so much pressure on healthy people now struggling to keep their jobs when competition is so fierce. I could imagine showing any form of weakness could make our job security even more unstable.
If there's anything you enjoy doing as a hobby, is there a way to convert it into a paid income? Being your own boss although hard work suits a lot of people with ME and is sometimes the only way.
Its just really difficult to advise when each person is so different. I hope you're able to test the waters somehow first before making any big comittments. See if you can repeat something weekly first and build from there.
Good luck with whatever you decide to try