To make a separate assignment for one or a handful of students, you will first create an assignment as usual, with the following exception.
Recommended: Include "make-up" or something similar in the assignment name to differentiate.
Include a Password as part of your Settings (Timing screen)Click Advanced Settings below the Time Limit setting
Once the make-up assignment has been taken by the student, you will manually enter their grade in the gradebook under the column for the original assignment. For instruction, see the Override Grade (Assignment as a Whole) section of the following User Guide article: Override Grades
You will then need to reset the points for the make-up assignment to zero* so students overall course grades are not penalized for the version not taken. There will still be points/marks values associated with each question. To reset points see the Entire Assignment section of the following User Guide article: Grading
*Set to 0.1 if you want to allow the student taking the make-up exam to be able to see selected Review Options when reviewing their attempt.