2) Locate the map in the database that you'd like to import to ArcGIS; the maps can be filtered by location and date. Note that for the map to be imported directly into ArcGIS Online, it must already be georeferenced within David Rumsey (indicated by the orange "View in Georeferencer" button).
3) Click the orange "View in Georeferencer" button and the map will open up as an overlay. From here, click the orange "What Next" button in the bottom-right corner of the screen followed by "Go to this Map page" button.
Same map viewed in the georeferencer; click the "What next" button.
4) A page containing map metadata and other information about the map will appear. In the section on the left called "Use in GIS apps," click on "Get links" and then copy the URL under "XYZ link".
Use XYZ Link URL
5) Return to ArcGIS Online and sign in with your free account. Once signed in, click the "Map" button in the top menu bar to open a blank map.
Click the "Map" button to create a blank map
6) To add your map from David Rumsey, click the "Add" button followed by "Add Layer from Web".
You want to add a layer from the web
7) This will open the add layer from web dialog box. When it asks what type of data you are adding in a drop-down box, choose "A Tile Layer". This will expand the box and create a place for you to paste the XYZ URL from David Rumsey. Do so, and add the proper title and credits for your map.
Paste your URL and include Title and Credits
8) Click "Add Layer" and the layer should be added to your map (you may have to move to the location to see it depending on the size of your map. Save by pressing the Save button in the top menu bar. Finally, make your map public by clicking the "Share" button and choosing "Everyone (public)".
9) Now it's time to add it to an empty StoryMap! Go to https://storymaps.arcgis.com/ (it may or may not ask you to log in with the same information as ArcGIS Online). Create a new blank StoryMap by selecting "New Story" --> "Guided Map Tour"
10) In your new blank tour, click the "Map Options" button in the top right corner of your empty map, then "Select Basemap" in the left-hand pane that opens up, followed by "Browse More Maps". This will open up a window showing all your self-created maps, including the one you just created
Select Map Options....
...then Select Basemap...
and select the map you chose from DR and it will import it in!
Now you can start adding points to your map tour on you historical map!