Large-scale experiments are building blocks of the physics community: they involve a large fraction of the scientific staff working in multiple countries, and absorb a significant volume of the science budget. They are also a collection of carbon-emitting sources and practices. As such, it is essential to assess their environmental impact. We describe here a methodology to estimate the main greenhouse gas (GHG) emissions of a large-scale astrophysics collaboration project, using transparent open data. The goal is neither to consider all possible emission sources of a project, nor to calculate accurate values. It is rather to identify the biggest emission sources of the project, obtain orders of magnitude for them and analyse their relative weights. We discuss methods to quantify the GHG-generating activities and their related emission factors for the three typical biggest emission sources that can be controlled by the collaboration: travel, digital and hardware.