If you know the new destination of all applications, you can start planning. You do this with a roadmap, in which, in addition to the technical activities, you also bring together the organisational activities and the impact on the business in one clear diagram. For example, different people are involved per application move, which will temporarily disrupt their daily activities or create the need to retrain. Therefore, always make an inventory of the human resources in your company. What impact will your cloud migration have on them? And is there enough in-house knowledge or do you need to bring in an external party to guide you?
Based on this inventory you can plan the actual migration. Make sure that the pressure is well distributed throughout the organisation. Then first migrate the applications that give you the fastest results in their new cloud form. Great examples are non-mission critical applications that are far more cost-effective as a SaaS product.