Answer: it’s not really easy. Abstraction has at least two levels. The fact of saying ‘tree’ is already some kind of abstraction as it implies that the mind recognizes the same in the same and the essential difference from others. (Is said essential this without what it can’t be, and accident that what is to it, but it can be without: a tree has necessarily a trunc – essence – but it may be white, or brown, or green – accident.)
What does this mean? Abstraction refers itself to formal patterns, here: same / different, and to formal patterns of understanding (essence, accident, all, some, consequence, etc.). To think abstract you have to use these formal patterns inside of a logical frame determining the rules that warrant intelligibility (to speak kantian): to say – the essence of a word does determine the logical validity of a consequence.
If it rains I become wet is true only and only if essentially rain is made of drops that are made of water, as to become wet is defined as ‘get in touch with liquid’, thus the consequence is drawn through some characteristic that is put in relationship to b through some logical mean.
Consequence: it is very difficult to think abstract, and even more ‘a priori’, but that will be the subject of another search.