Lord Shiva is known as Ashutosh ,meaning easily pleased but there are many rituals for worshipping a shivlinga like use milk to bathe the lingam, not to consume the fruits offered to him as Prasad, always keep the lingam under continuous water flow ,sprinkle gangajal over yourself before puja ,don't use kumkum or turmeric or ketaki flower etc.Are these necessary to please him or these are just manmade ?
These rules are mentioned in some scriptures (i.e the ShAstras) or the other. So, you can take them as the orders of God himself..
Now, it's upto you to decide, whether by not following the orders of God you can please him or by following them..
For example, that Ketaki flower should not used in Shiva Puja, is something mentioned in the Devi BhAgavata PurAna:
Never worship Visnu with rice; Ganes’a with Tulasî leaves; the Devî Durgâ with Durba grass and S’iva with Ketakî flower.
From the PurAna's Book 11, Chapter 17.
So, these rules are certainly not man made. And you can now decide what to do with them.
In Bhagwad Gita, Lord Krishna had said that anybody who worships me with utmost devotion, I will be pleased by their worship. It doesn't matters what he or she was offering to me. Patram pushpam phalam toyam yo mada bhakta prayachhati. Similarly Shiva was pleased very easily because he was also known as ashutosh. You can only offer water and belpatra on shivling & that's all, your worship was completed. But your devotion matters mainly.