Most often, feminine rhyme is created by turning an iamb into an amphibrach,* so that an iambic line ends with an extra syllable. (So syllable-counting won't work. A line of iambic trimeter is usually thought to have six syllables. A syllable-counter would see these lines as, perhaps, catalectic, having seven syllables rather than six. Syllable counting misses the flexibility of variable feet within a metrical line.)
The example given in "Lost in the Funhouse" is
What's good is in the Army;
What's left will never harm me.
Barth prefaces the quoted lines with the words "Ambrose's mother sang an iambic trimeter couplet from a popular song, femininely rhymed:". (Barth does not mean that something about the couplet is somehow effeminate. "Feminine rhyme" is the technical term for such verse elements, as stated above.)
Masculine rhyme consists of rhyme in which the final syllable is stressed. To turn the above into an example of iambic trimeter ending in a masculine rhyme, one could change it thus:
What's good is on the farm;
What's left will never harm.
* An iamb is a foot composed of two syllables, with the first syllable unstressed and the second syllable stressed: "What's good". An amphibrach is a foot composed of three syllables with only the middle syllable stressed: "the Army".