You can grade assignments even if students have not submitted anything:
Can I grade work that’s been done offline outside of Moodle?
Yes. Just untick all submission types when setting up the assignment and just use it for grading.
The quickest way to do this is, unexpectedly, to use Quick Grading. When you enable this, you will have input forms directly in the grading table. Just enter the grade in front of each students and click on the button at the bottom of the page to save.
(If you want to get complicated you can use CSV imports and whatnot as explained in Solar Mike's answer using Offline marking, but honestly why make it complicated when you can make it simple...)
Another, maybe simpler possibility, is to simply add Grade items and then edit them directly into the Gradebook. For something so simple it probably doesn't make much sense using the full-blown Assignment activity, unless you want to add material each day like grade sheets, solution sets etc. So you first add Grade items as explained in the doc, and then edit them in the Grader report. It can be done very quickly.