Learn Spanish (Latin America): Busco el diccionario en mi mochila. - I'm looking for the dictionary in my backpack.

The verb buscar already means "to look for" all by itself. You don't need to add an extra word like por or para after it. Saying busco el diccionario is basically like saying "I seek the dictionary."

How would I use this lesson to say "I'm looking for THIS dictionary"?

You would replace el (the) with este (this), making the sentence: Busco este diccionario en mi mochila. We use este because diccionario is a masculine noun.

And how would I change the ending to say "in THIS backpack"?

You would replace mi (my) with esta (this), saying: en esta mochila. Because mochila is a feminine noun, we have to use esta instead of este.

Why doesn't the sentence start with yo (I)?

In Spanish, the ending of the verb tells us who is doing the action. The -o ending on busco already means "I look for." You can add yo at the beginning if you want to be extra emphatic (Yo busco...), but leaving it out is the most natural way to speak.