The essence of legal malpractice is to show that a lawyer improperly did, or failed to do, something specific that, had the lawyer done properly, would have resulted in a measurably better outcome. Sounds simple enough, but it gets more complicated in practice.
First, consider the specific act or omission. It's not enough to say a lawyer "did not do a good enough job." You need something concrete. Typical examples include missing a statute of limitations, making a false statement, or waiving an crucial objection. Sometimes there are grey areas, such failing to properly investigate the facts of a claim or inform the client of legal consequences of making certain claims (e.g., attorneys' fees exposure in the event of a loss). These cases present special considerations beyond the scope of this blog entry.
Second, consider what should have been done. That is, you need to show what a "reasonable attorney" would have done in the particular situation. While this typically requires expert testimony to establish the standard of care, sometimes expert testimony is not required when the breach of care is obvious, such as when someone misses a statute of limitations. In addition, ethical rules governing attorneys can often be used to establish the standard of care or support testimony concerning the standard of care.
Third, consider the consequences of having things done differently. Can you really show the result would have been measurably different? How are you going to prove that to a jury? Understand that this is the "damages" element of the claim - that is, the difference between what happened and what should have happened is your damages. How will that be calculated?
These are just some of the initial considerations in a legal malpractice scenario, but as you can see, it is often more complicated than it initially seems.