Shri skandha puranam which is one of the 18 major puranams describe eight vratas as the pleasent vratas for Lord shiva. The puranam also prescribes the way in which these vratams could be observed. Please note that these are suggestions to observe these vratams taking the best of the ancestors knowledge and ritual traditions and not mandatory that it has to be observed only this way. Ultimately the sincerity of worship and the love for the Lord is what would bring most of the benefits. So the puranam itself states that "One who observes these great vratams along the prescribed guidelines or otherwise, they get the boons and finally get led to the liberation."
In these vratams it is said that the idol of the Lord be made and worshiped in relevence to the vratam being observed. In quite some places it is mentioned in the descrition of these vratams that it is for the people who do not do regular worship of Lord shiva with a Murti (lingam or other forms). Those who do the regular worship with a Murti could do the vrata worship also with the same Murti.
The Ashta Maha vratams are: