Adding Flags to all individuals in a project


I am on the point of merging two projects which share about 20% of individuals. I want to flag every individual in the smaller project with an indicator flag that they originate in that particular project. I have created the flag but cant work out how to apply it to every individual without having to do so one individual at a time. I,m sure there will be a way to a "add flag to all individuals in project" and I,m sure its dead simple when you know how, but I cant see it at the moment? Could anyone illuminate me?
thanks in advance

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