SendNextMonday

Step 1: Enable the Developer toolbar in the Outlook Ribbon

To enable it for Outlook 2010 and Outlook 2013:

Press the File button next to the Home tab and choose Options.
Select the section Customize Ribbon.
In the right pane, enable the selection field before “Developer”.
Press OK to close the open dialog.

Once you have enabled the Developer tab, select the tab and press the Macros button. This will launch a dialog with all your macros to choose from. Select the one you want and then press the Run button to execute it.

Step 2: Copy the code into a new Macro

Go to the Developer Tab and open the Macros editor Get the code from "SendEmailNextMonday.bas" into the new Macro Save it. You can rename it whatever you want.

Step 3: Add a Ribbon button to run the macro

In Outlook 2010 and Outlook 2013, things have been made a lot easier as it allows you to manually customize the Ribbon as well.

Open the item window where you’d like to add the macro button to.
Select the tab where you’d like to add the macro button to.
Click the File button next to the Home tab and choose Options. Here, select the section “Customize Ribbon”.
or
Right click any tab and choose “Customize the Ribbon”.
You’ll find the current tab already highlighted and selected.
Create a New Group or even a New Tab to place your custom button in. You cannot add your button to an existing group.
You can use the Rename… button to give your group and/or tab a nice name.
From the dropdown list “Choose commands from:” select: Macros.
The list below will now show you all your macros.
Select the macro that you wish to create a button for and press the Add >> button.
To modify the name and icon press the Modify… button.
    Unlike a toolbar button, you cannot assign your ALT+key combination via letters.
    When you close the Editor Options dialog and press and release the ALT button on your keyboard, you’ll a letter above the tabs. Type the letters above your tab and then you’ll see letters above the the commands on the commands. Type the letter above your macro to activate your macro.
    Unlike a toolbar button, you cannot paste your own icon or freely edit it. However, the icon list is longer and the icons are of a higher quality when compared to previous versions of Outlook.
Close the Editor Options dialog to return to your item window and use your button.

The above steps 1 and 3 are explained better in the original instruction page here: https://www.howto-outlook.com/howto/macrobutton.htm