The explorer maps are perfect for this kind of navigation.
They are actually free online via bing maps: https://www.bing.com/maps/
Select the Ordinance Survey layer and zoom in.
Personally I pay for the subscription as it's about £30 a year and means I can use the app.
That said when I go hill walking I take a printed out paper copy with a compass as a backup. It doesn't usually take up too much space if you print along your planned route.
How I did it using just a smartphone:
Use a tool that best fits you to draw a line (Google Earth, Google (My) Maps). Just check beforehand that you can export the line to e.g. a kml file.
Us Google My Maps to create a new map and import the kml file. You can then share the created map and open it on your phone with the google maps app.
When you are out in the field, you'll see the line and where you are on Google Maps. To track your route, use a GPS tracking app like GPS logger (https://play.google.com/store/apps/details?id=eu.basicairdata.graziano.gpslogger). Just let it run in the background.
Afterwards you can export your tracked route and import it again on your Google My Maps Map. You can measure deviations and so there.
I hope that helps. Maybe you find some more info here: https://www.reddit.com/r/GeoWizard/comments/marxrd/the_mission_across_lichtenstein_platinumgold/
How I did it using just a smartphone:
Use a tool that best fits you to draw a line (Google Earth, Google (My) Maps). Just check beforehand that you can export the line to e.g. a kml file.
Us Google My Maps to create a new map and import the kml file. You can then share the created map and open it on your phone with the google maps app.
When you are out in the field, you'll see the line and where you are on Google Maps. To track your route, use a GPS tracking app like GPS logger (https://play.google.com/store/apps/details?id=eu.basicairdata.graziano.gpslogger). Just let it run in the background.
Afterwards you can export your tracked route and import it again on your Google My Maps Map. You can measure deviations and so there.
I hope that helps. Maybe you find some more info here: https://www.reddit.com/r/GeoWizard/comments/marxrd/the_mission_across_lichtenstein_platinumgold/