The answer is in the other thread. Even a sample file.
Make sure you have allow creation of records via relationship selected on both sides of SignUps TO.
On the student layout add a portal to signups and put the SignUps::Course_Id field in the portal.
Create a value list using Courses::Course_ID and showing only Courses::Course_Name as second field in value list.
Change the portal field from edit edit box to PopUp and use your Course value list.
Go to the last portal row and simply select a course name.
Thank you. I have tuition fee as one of the column in Courses table. How to display the total tuition fee for the selected courses.
Depends on where you want to display it.
There is a lot of info on this sort of thing. Please try searching the community for Display Sum of a Field or something like that.
A summary field defined in Courses could be used on your Students layout to show the total of selected courses.
A calculation field defined in students could use the sum function to sum this field from Courses to show the same total.