To say that you are a poor cook【主语】// is 【系动词】//to do yourself an injustice【表语】//.

句子整体框架结构：A is B【To say sth is to do sth 说某事就是做某事】

A=to say that you are a poor cook 【不定式to say+that宾语从句】

B=to do yourself an injustice【不定式to do+间接宾语(yourself)+直接宾语(injustice)】

备注：do sb an injustice=do an injustice to sb 对某人不公平/不公正

We may have been doing him an injustice. This work is good.我们可能冤枉他了。这工作干得不错。

The article does them both an injustice.  =The article does an injustice to them both.那篇文章对他们俩都不公。