I've done this for brochures and other literature, and it's a bit of extra work compared to marking up a PDF with circles and arrows. But some publications have had problems with people using different PDF readers and these marks appearing in the wrong locations, so there may be a reason for this.
"Describe the action" means you should let the proofreaders know what text you need changed, and to what you want to change it.
Location: First you need to describe where the text is. As indicated in the instructions: page number as well as line number/column number, and so on. If there can be any doubt whatsoever about the location you mean, it's always best to give more information than less. (You seem to be following this part just fine.)
You may want to keep in mind that "paragraph X" can be ambiguous. If a page starts in the middle of a paragraph, does that paragraph count as the first one on that page? If they give you no instructions about this, you can include a note in your email indicating how you're doing this. Be consistent.
Page 12, paragraph 3
Page 12, column 2, line 21
(Line numbers are wonderful to use when you have them.) If the location of text you're describing is on more than one line, you can provide a range:
Lines 18 - 19
If the location you're describing is on the very top (or bottom) line of a page, it should be safe to just say that: "Page 32, first line."
Numbering in English is done with one on the top (or on the left in the case of columns).
Describing the change: This is much easier to explain with examples.
[Location] "And in the end, the love you fake" should read "the love you take"
[Location] "Jerry was was a racecar driver" - the word "was" is repeated, please delete the extra word.
[Location] Section title "Teh End" should read "The End"
Bolding text to be changed is helpful, but some email systems may not preserve this information, so don't depend on that.
Quoting article text like I did above helps find changes in an electronic file simple. It's easy to do this if the PDF you have allows you to select a bit of and copy it, then paste the text into your email.