Worried that you're not worthy? Well, worry no more. At Toasted Cheese, our motto is, "If you can read, you can critique." Remember the majority of reviewers, editors and agents are not writers themselves, they're readers. They read and evaluate. And so can you!
First, stop bashing yourself for perceived deficiencies. If grammar is not your forte, don't stress yourself out trying to fix someone else's. Instead, comment on something else -- plot or character, for example. Use your strengths.
Next, ask yourself some basic questions about the piece you're critiquing. Be honest. At this stage, the critique is for your eyes only.
Did you like it or not? Why? List some concrete, specific positives and negatives to help you organize your thoughts. An example of an positive would be: "Your dialogue was believable. It sounded realistic." An example of a negative would be: "The 'moral of the story' line at the end was too preachy and obvious." (Stuck? Check out the list of critique questions for ideas!)
Would you like to read more of the story / this writer's work? Why? Here you might say: "I'd like to read more because the main character was fascinating." or "I don't want to read more because the pedantic, passive voice writing style made me feel like I was reading a term paper, not a story."
What changes would you suggest to the writer? Why? Some examples: "I'd clean up the spelling mistakes and typos because I found them really distracting." to "I'd use 'she' in place of the narrator's name at times, because using her name every time is annoying, especially when she's the only person in the scene." to "I'd change the scene where the main character jumps off the cliff because that wasn't realistic since he'd just declared his love for life in the previous scene."
Finally, organize your notes so that you can show them to the writer of the piece. The purpose of a constructive critique is to help writers improve their writing, so it's important to be honest. Don't say a story was "Great!" if it wasn't. If the writer knows their story wasn't great, they won't ever trust any feedback you give and if they happen to think their story *is* awesome, you're just setting them up for a bigger fall when they send their not-so-great story off to a publisher before it's ready. At the same time, a critique should avoid being cruel, so even if you thought a story sucked out loud, please don't say, "This really sucked!" Instead, concentrate on specifics and on finding solutions for problems.
Start off with one or more of the things you thought the writer did well. Remember, saying something positive *is* possible, even if overall, you really didn't like the piece. Then, share the things you thought detracted from the story along with your suggestions on how to fix them. Suggesting how to fix the problem is what makes it *constructive* criticism, so do try to give a suggestion for each. Wrap your critique up with a positive statement or two, for example, "I can't wait to read more!"
Just like any skill, the more you critique, the easier it gets. As a side benefit, critiquing can give you valuable insight into your own writing, so give it a try! At Toasted Cheese, you must critique at least two pieces for every piece you post at a critique forum.