Begin by identifying and categorising the successes of the Republic. The four aspects (land, church, army and regionalism) identified by Culpin and Henig (226-7) are a good place to start but they need to be supplemented  by Forest (18-20) who identifies other socio-cultural successes. 

The second aspect of your plan should consider why the above policies were unpopular with particular groups in Spanish society. It is easy to understand why landowners or the Church would oppose the changes. The difficult part is to understand why many on the left opposed the government. It may help you to think back to why Russia's  Provisional Government of 1917 failed. At all stages of this plan, remember the importance of supporting your assertions with usable facts.

On the Historiographical issues raised by this question see Knight chapter 2.