Italian seasoning is a mix of dried rubbed herbs that are “characteristically Italian.” You buy it already mixed in spice bottles. Generally, the mixture will include some or all of the items that follow: basil, coriander (sic), marjoram, oregano, rosemary, sage, savoury, thyme.
There is even a mix called “New Orleans Italian Seasoning” (sic.)
Many cookbooks and web sites will offer various opinions of what should be in Italian seasoning, and truth to say, there is no official version of what goes in the mix.
To be clear, all herbs mentioned are in “rubbed” form, not ground into a fine powder.
McCormick, the spice company, suggests for 1 teaspoon of Italian seasoning use “1/4 teaspoon EACH Oregano Leaves, Marjoram Leaves and Basil Leaves plus 1/8 teaspoon Rubbed Sage.”  McCormick. Substitutions. http://www.mccormickforchefs.com/Links/Cooking-Resources/Substitutions. Accessed November 2016.
The University of Delaware Extension Service lists 7 ingredients. They call for equal parts of “Basil, marjoram, oregano, rosemary, sage, savory, thyme.” Snider, Sue. Herbs & Spices. What goes with what food. The University of Delaware Cooperative Extension. Fact Sheet. August 2012. Accessed November 2016 at http://extension.udel.edu/factsheets/herbs-spices-what-goes-with-what-food/
As a quick and perfectly fine substitute, just use basil and oregano, equal parts.
Based on the University of Delaware’s ratios, to make nearly 1/2 cup (20 g / .7 oz) so you can have some on hand:
1 tablespoon rubbed dried basil
1 tablespoon rubbed dried marjoram
1 tablespoon rubbed dried oregano
1 tablespoon rubbed dried rosemary
1 tablespoon rubbed dried sage
1 tablespoon rubbed dried savoury
1 tablespoon rubbed dried thyme
(Yes, the 7 tablespoons does fluff up in volume to be 1/2 cup.
Store in a sealed jar, container or bag out of the light. Label and date.
|↑1||McCormick. Substitutions. http://www.mccormickforchefs.com/Links/Cooking-Resources/Substitutions. Accessed November 2016.|
|↑2||Snider, Sue. Herbs & Spices. What goes with what food. The University of Delaware Cooperative Extension. Fact Sheet. August 2012. Accessed November 2016 at http://extension.udel.edu/factsheets/herbs-spices-what-goes-with-what-food/|