Most important thing to do is read the publication, get to know it and decide where your article fits. Go to
http://www.guardian.co.uk/weekend
There's a generic email address - weekend@guardian.co.uk but I personally would find out who the editor or features editor is (depending on what you wanna send) and send direct to them (unless it states otherwise on website). You can usually get their names by looking in a copy of the publication or googling it.
This is where I may need others to pipe in as been a while since I edited lifestyle mags / newspapers (editor in HE / web now) so others might have more current advice. I would email them with a quick overview of your writing experience (what you've had published with links if poss), your rates (if you charge - NB. they will only take something on, usually, if you have published material already out there) then a quick overview of the piece and why you're the right person to write it (what experience you have in the field you're writing about). I'm not sure how the Guardian feel about attachments - it might be better to ask if they want to see more before attaching.
Good luck!