The process starts from creating project in LCS and Azure dev ops and connecting them. How to get access and create project for D365 for finance and operations in LCS is well defined on the link shared above and link below .
docs.microsoft.com/.../support-experience
High level steps for code upgrade once you connect projects from LCS and Azure dev ops are
- You are going to upload model store from AX 2012 to LCS to get the estimates
- Once you get estimates , run actual code upgrade service from LCS , which s going to check in all your code as Overlayed code. All code is going to be in XML artifacts.
- You are going to deploy VM's for developers and map their VM's to branch created by code upgrade service in Azure dev ops.
- You are going to remove all over layered code and refactor it using extensions . More information about extensions is present on extensibility page
Refer my below blog Post as well
us.hitachi-solutions.com/.../upgrade-dynamics-ax-code-dynamics-365-operations
us.hitachi-solutions.com/.../plan-upgrade-dynamics-ax-code-dynamics-365-operations