On Guru Pournima we worship the formless God. Worship is offering our Love and respect to Him. You may do so by Meditating or channelling Light. We have to be aware of His presence all the time and try to experience Him. We can experience Him as Infinity, Vastness, Love, etc. If we can remain in this experience then that is same as worshipping God.
Being in communion with God with great love, all through the day is how we observe Guru Pournima. If people rush to Ashrams and offer respects to their guru that is fine. Because God is the real Guru and human guru represents Him.
The Spiritual part of Guru Poornima is that there is a strange alignment of the Sun and Moon on that day, due to which a lot of energies come from the highest Source. The whole earth is filled by that Power. This process begins from the first day of Ashadha Masa and reaches its peak on Guru Pournima Day.