Every situation is different but here is a typical running order for the reconfiguration of a listed flat:

Chat with the listings officer to give you an idea of what is allowable
Check the lease conditions (with solicitor)
Obtain agreement in principle with leaseholders/freeholders
Commission the designer to produce planning drawings
Apply for planning permission (as it is a listed property)
Appoint a structural engineer
Check underground services with Utility companies
Comply with Party Wall Act requirements
Get quotes from contractors
Apply for Building Control approval
Start work