I am flying Ethiopian business class to Cape Town in a few months so I thought I'd take the chance to start earning points on Star Alliance. This will be my first flight for ages with them. Can you still redeem from a flight 2.5 years ago?
I was having a quick skim of the Star Alliance rewards page and I'm a bit confused:

"You can join multiple member airline programmes if you wish, but they are each managed separately so you cannot transfer miles or points between accounts or combine them. Using a single programme whenever you fly means you can earn privileges and reach Star Alliance Silver or Gold status faster. "
So do I just sign up to Star Alliance or do I go with a specific airline? the former how do I ensure that my points go through to star alliance?

Thanks

Different programs will allow for different past flights, but I've never seen anyone credit beyond one year, and typically much less.

You have to pick an airline program and sign up with it. The sticky at the top of the forum should help you decide.

You cannot "sign up to Star Alliance" as they do not have a frequent flyer program. They are a marketing alliance
You sign up with an airline ffp.
Airlines have alliance & non alliance partners
Where to Credit | Where to Credit Ethiopian Airlines Flights in J Booking Class
Aegean Airlines (A3) has been popular as "hold all" Star ffp, but has lost some appeal recently

General:- Which Frequent Flyer Program to Join? Help Is Here!

Oneworld:- Deciding on a Oneworld Frequent Flyer Program? Help is here]

Star Alliance: Need advice on which Star Alliance FF scheme to join? - please post here

Skyteam:- Which SkyTeam program should I sign up for?

Be mindful of expiry Miles/Points that Do and Don't Expire. Click to open the wiki.

Ah oops. I started posting in the Which programme to join then thought it was too general. Will go there.

Thank You