These two expressions are blunt hints at the stupidity or lack of truthfulness of the person being spoken to. Si serás (see-eh-RASS, lit. if you shall be) is the tuncation of si serás pendejo, which means "what a dumbass you are". Hazte (ASS-teh, lit. make yourself) is the truncation of hazte pendejo or hazte güey. It means "stop prentending you don't know what's going on".
Since many Mexicans realize what you mean, these two expressions shall be used with care. Do not thow them at people you don't want to offend.
Chalán: No se enoje. ¿Cómo iba yo a saber que no tenía aceite?
Mecánico: Si serás... Ora sí nos pusiste a parir chayotes con tu pendejada.
Assistant: Don't get mad at me. How was I supposed to know it needed oil?
Mechanic: What a dumbass you are. Now we're in deep shit thanks to you.
Fer: ¿Lápiz labial?
Liz: Don't pretend you don't know what I am talking about.