This only works if you had Google location history enabled during your trip. If you don't remember enabling it, it's still worth checking as you may have switched it on unwittingly.
If the routes shown are straight lines rather than your exact route, this could mean that your phone's location was switched off and Google was trying to guess your route. If that is the case, don't despair. You can edit this .kml file in Google My Maps (different from Google Maps) and patch up your route. The guide for Google My Maps is below. However, if large sections are straight lines, it might be more time efficient to re-create your route from scratch.
If you planned your trip on Google Maps and shared your directions link with your travel buddy, you can paste that same link in some online “google maps to .gpx” converters.
For example, Maps to GPX (https://mapstogpx.com/) is a pretty cool free online tool for that. Once converted, attach your .gpx file to your order form.
If you intend to recreate a route in hindsight for us, we recommend you use the Google My Maps method explained below.
Use this if you remember your trip itinerary but didn't record your GPS data during the trip itself. Follow below to create a route in hindsight.
If you have a .kml you wish to edit, perhaps an incomplete file from you Google Maps Timeline, follow the above guide up to step 2. After you've created a map, click the blue “import” below the relevant layer title. Follow the instructions to import your .kml file. Once the .kml file has been imported, edit and create your route as in steps 3 and 4 and continue to follow the guide to export the amended file.