Right now mdEditor is in Beta at https://my-beta.usgs.gov/mdeditor/
Right now you can enter data into the app either by hand or using a combination of a ScienceBase record and by hand data entry (See "Seeding a Record Using ScienceBase).
You can then save a copy of your work as an mdJSON file (See "Saving a Copy of Your Record as an mdJSON File).
If you save a copy of your mdJSON you can upload that file back into the app later and continue working on that item (See "Upload a Copy of your mdJSON File").
mdEditor allows users to seed (or start to populate) a record using an existing ScienceBase (SB) Record.
Your record will be populated with as much data from ScienceBase as the app can pull for you.
You can save your record at any stage in the data entry process by clicking on the "Save mdJSON" button under the SAVE heading, which will download and save a copy of your work to your computer.
If you save a copy of your mdJSON (See "Saving a Copy of Your Record as an mdJSON File) you can upload that file back into the app later and continue working on the item.
You will see that a copy of your file has been added to the app and the relevant fields have been appropriately filled out.
NOTE: If you make more changes, please remember to save a new copy of your work.