Missed words, typos and incorrect grammar imply that you don't care about your communications.
Not good.
Correcting misspellings and errors in your writing is crucial.
But most of us are desiring to do something else, when we stare at our darling sentences.
One building block to effective, accurate proofreading is tightly focusing your attention and slowing down.
First decide, should you proofread the text on your computer screen or on paper? On-screen proofing is fine for short notes, informal emails, etc.
If it's more than one paragraph, or important, print it out and proofread.
Tightly Focus Your Attention: Read What's There On paper

• Isolate each line of text.
  Place a ruler or piece of paper beneath the words.
  As you read, slowly move down the page line by line and make notes.
  This tactic slows you down and forces you focus on the text.
  
• Read the sentences backwards.
  For example, "The cat hates the dog.
  " would be read as "period dog the hates cat the.
  " Yes, it's laborious, but it's a fantastic method for discovering missing or repeated words and punctuation.
  

On screen

• "Zero in," on one sentence at a time using your word processor's "zoom" function; increasing text size.
  
• Use contrasting colors to make the letters stand out.
  Consider changing the background color of the "screen" or page, to a pale yellow or light blue (when the text is black).
  

On screen or paper

• Slowly read the words out loud.
  Along with missed words, this step pinpoints long sentences and awkward transitions between ideas.
  

Take the time to slow down and focus.
Save yourself embarrassment and the potential errors that just don't bear repeating.