Speaking from a high level, from a DW development perspective, firstly one needs to figure out the boundaries of development. Generally the extreme boundary starts from OLTP and ends at Dashboards. After having these boundaries considered, the following are the layers / development arenas one can consider for a DW development from scratch.
1) Delta Detection - This is the first exercise that you would plan with your OLTP system. This is a very important exercise, as this would decide a few other exercises in the life-cycle.
2) Staging Area - Based on the requirements and considering the complexity of delta detection, a temporary / permanent staging area development would be required.
3) Master Data Management - Do not confuse it with the standard MDM practice, which is more towards modeling. Here MDM means how you would manage your MDM in the staging area / in the delta, as delta applies only to transactional data. Master data do not change that often, and you need master data for your ETL processing. This has to be managed at the facade layer you would build for delta detection / in the staging area, but it has to be planned along with the points 1 and 2.
4) Dimensional Modeling - This is the exercise where you start modeling your dimensions using the Kimball / Inmon methodology.
5) Data Mart Design & Development - Only after the point 4, one can start developing a data mart which lays down the base for the next exercise of ETL development.
6) ETL Design & Development - Points 1 - 2 - 3 are to serve the ETL processing. ETL basically serves as the processing engine to transform your relational data to suit the model of your data mart. One can consider the above points in E and this is T + L. Until you Data Mart is in place, one does not have any idea about the target schema, so ETL development makes sense only at this level.