This practical methodological guide aims to support practitioners, primarily future EaSI project promoters preparing a social experimentation proposal in the 2021-2027 programming period. However, it can also be useful for other stakeholders, notably those responding to ESF+ national and regional calls on social innovation under shared management. In addition, this guide can raise awareness of policymakers and funding organisations at all levels (EU, national, regional and local), in particular ESF+ Managing Authorities, about the value of social experimentation. They can use the results of already successfully tested social experimentations or design and launch new specific calls to test innovative solutions addressing their policy needs, which may help avoid some failures and associated costs. This should improve the design of policies, programmes, calls and funding mechanisms to support social innovations and social experimentations.