It is helpful to develop a resource bank of the foundational concepts which have been learnt in geometry over the preceding years. This will provide students with a toolkit which supports the development of their proof writing skills.
Initially, deliberately teach and give clear advice about writing the reason for a geometric statement. Provide a list of theorems which can serve as a geometry toolkit that can be referred to.
Work with your class to complete the reasons for each theorem and to develop abbreviations which will be accepted as evidence of knowledge of the appropriate theorem. Solutions and some suggestions are made on the Geometry toolkit: Solutions.
In general, students should be advised to write whole words unless there is a universally accepted abbreviation (e.g. \(\angle\) for 'angle'). There is no need to write out the enunciation of the theorem in full. The minimum standard should be writing sufficient words to identify the particular theorem.