Unfortunately, I do not think there is a built-in Tutorial guide for creating one of them, but you may find a video on YouTube about them. They can be quite useful though, so it is definitely something worth looking into. They can range from very simple to complex, so creating them can also be either somewhat simple or pretty complicated. You start with new feature that you create normally. But it is definitely important to keep in mind that you are creating the feature for an iFeature, because some stuff may need to be done a bit differently than usual. For instance, you will want the sketch plane, constraints, dimensions, and such to be as independent and modular as possible. You will also want to rename every Parameter that gets created for each dimension you place in it, so that you know which parameters are for which aspects of the sketch. If you need to base your sketch off of some projected model edges, keep those types of things as minimal as possible, and remember precisely which edge it is being offset from, because you will need to know that later. Then when you create the extrude feature, there will be parameters automatically created at that time too, for things like extrude extend, angle, and such. Find and rename those parameters too, so you know which ones are which.
Then on the Manage tab, go to the Author panel, and choose the tool named Extract iFeature. This is where you can choose the feature, its sketch, the parameters involved, the reference model edges involved, and provide prompt type descriptions and names to those items, and set the order in which those things will be asked for when you go to place the iFeature in another model. Make sure you are as clear as possible in this extraction process, so you will know what you intended a year from now. Make sure you save it in one of the designated locations, such as the 'Catalog' folder under your "C:\Users\Public\Documents\Autodesk\Inventor <VersionYear>\Catalog\", because that is where it looks for iFeature and sheet metal punch tools by default.
Once that process is done, and the iFeature file is saved out, you can open another model file where you would like to place that iFeature, and on the 3D Model tab, you may have a panel showing namd Insert (if not, you may have to show that panel), and it may have a drop-down list. Choose Insert iFeature, then navigate to where you saved that iFeature file. Then follow the prompts where it wants you to select those same edges on that other part for what plane to put the sketch on, what edge(s) to offset from, what parameter values to use (some may not need to be changed, and are retained), etc.
Wesley Crihfield

(Not an Autodesk Employee)