Treebanks are widely recognised as a necessary source of information in NLP as well as in Linguistics studies. In this paper we present and justify methodological principles and syntactic criteria to build a Treebank for Spanish: annotating only explicit information, constituents and syntactic functions and being theory independent. Previous work is also presented in order to account for taken decisions. The annotation process will be done in different steps so that each one of them is the input of the next. We present the basic guidelines of syntactic annotation and the boundaries of the work to be done in a first step: annotation of low constituents and surface functions. Moreover, some semantic information (subject type) is likely to be included.