The short answer is that if you are fluent/advanced in Norwegian and can read Bokmål, then you will be able to read Nynorsk without much trouble.
In the situation that you are an intermediate reader of Bokmål, then it will be helpful if you know the major spelling differences between the two, as covered here and here. After knowing those "conversion" tricks, then there is the small matter of knowing some grammar differences, as mentioned here and quoted below:
Nynorsk grammar is probably a little more complex than Bokmål grammar,
especially because (as noted by Magnus Itland) you must observe three
different genders in nouns (masculine, feminine, neuter), whereas
Bokmål allows the collapsing of the masculine and the feminine into a
Common gender, morphologically like the traditional masculine. For
instance, the suffixed "definite article" is -en for masculine nouns
and -a for feminine nouns, so a traditionally feminine noun like bok
(book) can in Bokmål become either boken or boka for "the book"; the
former variant sounds more literary or upper-class, whereas boka
connects more closely with the unpolished everyday vernacular of Oslo
and the area around the capital. In Nynorsk, the form boka is
obligatory, and carries no particular stylistic implications.