The most straight forward way to copy a group of records from one table to another--doesn't matter if tables are in the same or different files, is to perform a find or use GTRR to bring up a found set of the records to copy, then use Import records to copy them.
Could you share more details/exact steps, because if it was easy for me, I wouldn't have posted this topic.
Import records is documented in FileMaker Help. I figured you could follow those instructions.
Bring up your found set in your source table. This can be a performed find or with a Go To Related Records script step. If this table is in a different file, leave the file open and switch to your target file. This be part of an Import Records script.
If this is manual import select a layout based on the target table and choose import records from the File menu. If this is a scripted import, set up your import records step to import records from the source table. If the source table is in the same file as the target table, you go through the same steps to select the source table as you would if the file is from a different table. you just select the file you already have open.
For both a manual import and a scripted import, you then need to set up the other options for your import, mapping fields from the source table to the target table as well as selecting other options. Sorry not to be more detailed, but there are a lot of options here to choose from and the right ones depend on the design of your tables and what exactly you need to do with your import.