After you have created and approved the contracts in Reviiri you can go to the "Summaries" tab on Reviiri and select the game you wish to get the export file for Oma Riista (tap on the Oma riista icon). The file will be downloaded to your computer.
After the file has been saved you need to go to Oma riista and import the file to Oma riista. On Oma riista maps tap on "Functions" and select "import from file" and select the file that was previously saved to your computer to export the maps.
Please note that there are some restrictions on Oma riista what can be imported and you can read more here.