How do you add Read more … link to posts?

How do you add Read more …link to a particular post or to all posts in a blog ?

Read more links usually appear under following conditions:

1. Template uses the_content() function for display.
2. Post has <--more--> tag in content, which creates teaser.

Result is teaser, followed by read more link.

Sometimes such links are created artificially by appending them to output of the_excerpt() function in template.

So to precisely answer your question – it depends on your template tags and how you use excerpts and/or teasers.

See post (by me) for writeup on excerpt/teaser differences and mechanics.