Can I make an ILS-Approach when I am VFR?
Sure you can.  Just request to fly the approach using either radar vectors or initial approach fixes, if available.  The tower will usually comply and grant you the approach clearance, adding "maintain VFR" i.e. The approach should be flown under visual flight rules and you cannot use a view limiting device unless you have an appropriately rated safety pilot onboard with access to dual flight controls.

For non towered airports, you will need to listen for traffic in the area on the CTAF and provide position reports often for the benefit of other pilots in the pattern.

Flying practice instrument approaches in VFR is quite common and many pilots do this to maintain instrument proficiency.  It is also common for instrument rated pilots to file a local IFR flight plan to practice as well.