What is the best way to adapt the template in InDesign® according to one's wishes?

Answer:

1. Open the IDML file in InDesign® and save it as a regular InDesign® file (.indd)
2. Adapt the size of the document by clicking in the menu on File / Document Setup and type in the dimensions
3. Make sure to accordingly adapt the text framework
4. Do you already have a wall calendar which was made with Adobe® InDesign® and you want to keep its appearance? Then import the paragraph and character styles by clicking on the character styles palette on Load Character Style or by clicking on the paragraph styles palette on Load Paragraph Styles. Then add the imported styles to the existing. No more steps are required.
5. Choose in the menu Type / Find Font. Replace the default font Myriad with a font of your choice. Check the field »Change all« also re-defines styles, to implement your font into the paragraph and character styles
6. Change the predefined colors in the color palette according to your wishes
7. Edit the paragraph styles for working days, Saturdays, Sundays and holidays according to your wishes
8. Edit the character styles for day numbers, week days and calendar weeks according to your wishes
9. Tip: You already have an InDesign® file from your last calendar? Just import the paragraph and character styles from your old calendar