This work is part of the TREMoLo project dedicated to language registers (casual, neutral, and formal). Here, we present an annotation guide grounded on a linguistic analysis of language registers and Computer-Mediated Communications (CMCs). It gives instructions for annotating French tweets according to the tripartition casual, neutral, and formal. First, it specifies and defines the elements specific to CMCs. Then, it presents the protocol for annotating tweets in language registers. All annotation choices have to be justified by at least one linguistic descriptor. The complete list of linguistic descriptors is presented with examples at the end of the annotation guide.