1. Install Portal and PortalRTX
2. Start Portal, press "Load game" to see if you have any savegames. Then close the game.
3. Start PortalRTX, start a new game, play a minute or two and press F6 to make a quicksave. Then close the game.
4. Lookup the savegame location of both games:
Portal: LOCATIONTOSTEAMFOLDER\Steam\steamapps\common\Portal\portal\save
PortalRTX: LOCATIONTOSTEAMFOLDER\Steam\steamapps\common\PortalRTX\portal_rtx\save
5. Delete everything inside the PortalRTX folder but DO NOT delete the "save folder".
6. Select all content in the Portal "save" folder and copy it to the PortalRTX "save" folder.
7. Boot up PortalRTX, press "load game" and see if your saves are transferred.
8. Enjoy!