Read the full text of The Merchant of Venice Act 3 Scene 4 with a side-by-side translation HERE.
At Belmont, Lorenzo is practicing his flattery on the ladies as usual, except this time it's with Bassanio's new wife—in front of Jessica! He compliments her for bearing the absence of her new husband so graciously and nobly. Still, he says, if Portia knew what a great guy Antonio is, and how good he is to her husband Bassanio, then she'd be even happier to do her wifely duty.
Portia says, "Shucks, it's nothing." Basically, if Antonio is such a great friend to her lord Bassanio, then Antonio might as well be her lord, too. Bassanio's absence is a small price to pay to get Antonio (and by proxy, herself) out of hellish cruelty.
Portia cuts to the chase and tells Lorenzo that she's going to go off to a nearby monastery with Nerissa to pray and contemplate for two days while the men are gone. In the meantime, Portia asks Lorenzo, "Will you house-sit my sweet mansion with all its servants and stuff, and basically be the lord of the house while Bassanio and I are out?"
Lorenzo generously says yes, and well wishes are made to Nerissa and Portia all-around from Jessica and Lorenzo. As Jessica and Lorenzo leave, Portia is left alone with her attendant Balthazar and Nerissa. She sends Balthazar on his way with some instructions: he's to take these letters to Padua and deliver them to Portia's cousin, a Doctor Bellario. The Doctor will likely give Balthazar some letters and clothes in return, and he is to take them and rush over to the ferry that goes to trade with Venice. She promises she'll be there waiting to meet him and then promptly rushes him off.
None of this has actually sounded like a plan to go to a monastery, and Portia announces cryptically to Nerissa that the two women will see their husbands sooner than they think.
Portia explains further: their husbands will indeed see them, but they won't recognize them. The women will be dressed convincingly as men. The other men (including their husbands) will think the disguised girls are accomplished men.
Portia is clearly going to have some fun with this one—she promises to be a prettier, daintier-looking boy than Nerissa.
Portia says she's studied many young fools and can mimic their foolish mannerisms quite convincingly. Such foolish mannerisms include boasting of the fights they've been in, bragging of the women they've spurned, and several other idiotic behaviors. She'll work convincingly to portray a recently graduated man who's a pompous idiot—as all young men tend to be.
Nerissa wonders what all the fuss is about and why they need to dress up like men anyway. Portia says there's a naughty way to answer this question, but it won't come from her. She says she'll explain everything in the coach, which is waiting for them. They have no time to lose, as they've got twenty miles and two drag king costumes to throw together in a jiffy.