para resumir

These transition words are useful to conclude your idea. 

En resumen – to sum up

En resumen, se trata de uno de tus mejores trabajos. 
To sum up, this is one of your best works. 

Para terminar – lastly, finally

Para terminar, debo aceptar que fue una gran experiencia. 
Finally, I have to accept that it was a great experience. 

En conclusión – in conclusion, in a nutshell

En conclusión, es la mejor decisión que he tomado. 
In a nutshell, it’s the best decision that I’ve ever taken. 

Other Useful Transition Words to Summarize:

En síntesis
In short

En otras palabras
In other words

Al fin y al cabo
All in all

Después de todo
After all  

If your goal is to be fluent in Spanish, transition words are a crucial area of study. For that reason, it might be a good idea to keep the following list at hand, as you will always need a good transition word to make your speech and writing sound more natural.