It is commonly used in the past tense in the following format:
"When (insert), I shat bricks (./!)"
where the (insert) is filled with a noun affecting/completing an action or being affected by a verb and the sentence is ended with either a period or exclamation mark (./!)
It is also utilized as a phrase of exclamation, also in past tense. This clause is preceded by a remark or sentence which could be reformatted into the form above. However, this form is not as prevalent as the abovementioned.
"When Bender saw it, he shat bricks."
Noun being affected by an object
"When that guy got hit by that snow plow, I shat bricks!"
"I shat bricks!"
Note: 'To shit bricks' is never used in its infinitive form and is conjugated in any of the twelve tenses