You set 1) number of document on the page. some pages have several of them. Set -1 if you wanna search through all documents
2) Number of form. The same logic as documents. Set -1 if you wanna search through all forms
3) Tag name that you're looking for. Definition of each tag you can find in any book or on-line course about HTML. Or Google it.
4) You set attribute of tag that you wanna use to filter necessary element among other elements with the same tag. ABout each attribure you can also get knowledge from any book about HTML.
5) Value of attribute that you will use as search term to filter elements.
6) Search type: text, regular expression, notext
When you set all these options for search (1-6) you hit Search (7)
In window 8 there will be displayed all elements on current page that fit your search terms.
Then you choose type of interaction: Set (add data to element), Get (Get any of it's attributes) or Rise (action like clicks, focus and so on)
9 - 10 you set option for Set / Get or Rise and in window 10 you can put value of attribute that you wanna set via Set option.
Then you hit Test and if you did everything right there will be action performed. If you see result that you wanted you click Add to project.
You must have basic knowledge about HTML in order to use action designer.