In the logs drop-down, choose 'user creation log'. Look for users who have made contributions but do not have a talk page - so, look for users with a redlink talk, but a link in contribs. (Don't welcome users that have not contributed; lots of accounts are created and never used, so no point cluttering up the wiki).
Also, skip any inappropriate user names (see WP:USERNAME) - consider reporting these on WP:UAA if appropriate.
Click their redlinked talk page, and create the page.
You can either use a standard welcome template, or use my own.
If you use a standard template, you should make a new section with == Welcome ==, then, on the next line, put {{subst:welcome}}
If you use mine, it creates a section automatically, so you just put {{subst:user:chzz/w}}
Put a suitable edit summary, such as 'welcome', and save the page
