How to write macro's

Hi Eelco, welcome to the forum, thank you for your post; the writing/editing and using of macros is covered comprehensively in our Excel Advanced and Excel VBA courses.
But to answer your question; a macro is a recording of mouse clicks and/or of keyboard entries which are recorded by Excel and played back (like a tape recording) whenever required, a macro is usually recorded to undertake mundane or repetitive tasks.
Unless you are employing VBA you are not really writing macros, you are recording them.
To record a macro, go to the Menu command Tools>Macro and choose the option Record New Macro, name your Macro in the dialog box and observe Store macro in: drop down box, this gives you three choices, if you wish your macro to be available to you in every workbook you open, choose the Personal Macro Workbook option, the other two are self explanatory. Observe the little floating toolbar which opens, the option to have your new macro work either Absolute (always in the same cells) or Relative, (within the same pattern from a chosen cell, but anywhere in the worksheet.) Undertake the task you wish to record, when you have completed the task, click the Stop Recording button (the little square, usually blue) Excel will remember all your mouse clicks and key presses, and replay them when you go to the menu command Tools>Macro and choose Macros, select your named Macro and press the button Run. This action can be automated by creating a button either on screen or on your toolbars and assigning the chosen macro to it, or by creating a Keyboard shortcut and assigning the macro to that. I hope this has helped, if so please click the Resolved link, best regards Pete.