Two variations on a theme.
1. A short list that doesn't change:
Put the list directly into the source box.
2. A longer list, or one that varies:
Put the list onto your worksheet somewhere convenient (you can hide the row or column if you want). This can contain formulas if necessary. I have one that has a formula to calculate Monday's date in B101, then =B101 + 1 in B102, and so on down to B107.
Select the cell(s) you want to have the drop-down.
Choose Data, Validation from the menu. A dialog box will open. Choose list from the Allow: listbox.
In the Source box put:
Method 1 - Monday, Tuesday, Wednesday, etc - or whatever. The commas matter.
Method 2 - =$B$101:$B$107, for example. You can click the colourful box on the right then select the range, if that's easier.
Make sure the in-cell dropdown box is ticked, add your messages and alerts, and click OK.