Is plastering a job best left to the professionals?
If you have failing plaster and lathe interior walls, the process is a bit more complicated than just troweling on some new top coat plaster. The scratch coat, or first coat that bonds with "keys" through the lathe. When these keys fail or break off behind the lathe, portions of the plaster will become loose and often fall off the wall.  The correct fix is to remove all the compromised material exposing the lathe and apply a new scratch coat creating new keys between and behind the lathes.  After that coat cures, then the finish coat is applied. The materials used are completely different for each step. Although it is not a very difficult job, like anything else that is very visible, the quality of the bond and finish are important to the structural soundness and visual appeal. 

True plastering is becoming a lost art in a world dominated by sheetrock. Plastering is one of those jobs that takes a long time and lots of practice to become proficient at. I certainly wouldn't discourage you from trying, but don't expect perfection on your first attempt. Even though the finish coat is what you see, pay special attention to the scratch coat as that is what holds the surface to the lathe. If the scratch coat fails, a perfect finish coat is worthless.  If your project is fairly small, go for it, but if you are looking at a whole room or rooms, it might be worth your time to get an estimate from a pro. With all the time and money you will spend doing a large project yourself and suffering through the learning curve, you may decide your time is better spent on doing something else.