How to message users?

not on here best thing is to try to message them through Steam

Reply to a message if they posted one or maybe leave a message on an old giveaway they created

You have 7 days to send the gift, nothing to apologize for, unless by a few days you mean a few weeks lol.

If you want to write to the winners, then you can write them e-mails or try to add them on steam, but that works 50/50.

there is no direct/private messaging on SG
what you can do on SG is

• comment on one of their old giveaways
• reply to one of their comments in an other win of theirs, like a thank you message
• reply to a comment of theirs in the forums though unless they made their own discussions you may no find an old comment from them on the forums and i would do also not to reply to a discussion that is already dead to not necro that post.

other than that

• you can email them. as a gifter you can see the email of your winners
• try to reach them over steam... but be very carefull with that as you might trigger steams algorithm for spam/scams when you try to leave a message on their profile containing words like gifts or win/won

but like aez76 said, unless its really been a long time since your GA ended, there would be no need to apologize

life happens :O
and as long as you deliver within the 7 days, there's no need to apologize ;)

If you want to send a private email to a winner, you can use Sendity, where the one who receives your email cannot see your address.
https://sendity.org/

In case if a user would not respond to your message, it will be a good idea to select an option for sending a copy to you.

The best way to reach someone on SG is to leave a comment on one of their old giveaways. If they haven't created one then reaching out to them on Steam or even there email will be the way to go. It's only been a few days so don't stress too much.