English equivalent of Polish idiom "To explain [something to someone] like a cow in bound"

Another Polish saying "Tłumaczyć jak krowie na miedzy" that means "To explain [something to someone] like a cow in bounds".

Explanation
This idiom is used to describe a person that is dumb above the average and/or has problems with understanding certain issue and you try to talk to him as plain as possible to explain it to him. Cow is believed to be a very stubborn animal and it is difficult to make it do something you want it to do - that's an etymology of an idiom.

More details

In original Polish version word "bound" stands for a stripe of grass between fields (country land) that marks the real borders of a field so that a farmer would know where his field ends and start his neighbours.

Example
- Wait, what was that bipolar junction transistor doing once again?
- Jesus, you're deaf or what? I've been talking to you like a cow in bounds for last 3 hours.

Little help
Picture that might help in understanding idiom

I can’t think of any animal-based idioms for this but maybe you could mix it with a well-known English one that is similar in meaning (the mixing part in brackets is a joke, btw!):

It’s as if I’m talking to a [cow stuck behind a] brick wall.

talking to a brick wall
Is a phrase used to sarcastically explain that the person you are talking to ... is not listening or is so dumb that they don't understand
Wow you're so stupid it's like talking to a brick wall

(from ‘Urban Dictionary’)

...I've been spelling it out for you for the last three hours

spell out

To give a detailed and literal statement of something in order to make it perfectly clear and understandable: The committee demanded that he spell out his objectives. She didn't understand the penalties at first, but we spelled them out for her.

(The American Heritage® Dictionary of Phrasal Verbs)

Do I have to spell it out?

Do I have to spell it out (for you)? and Do I have to paint/draw (you) a picture?; Do I need to paint/draw you a picture?

Fig. What do I have to do to make this clear enough for you to understand? (Shows impatience.) Mary: I don't think I understand what you're trying to tell me, Fred. Fred: Do I have to spell it out for you? Mary: I guess so. Fred: We're through, Mary. Sally: Would you please go over the part about the square root again? Mary: Do I have to paint you a picture? Pay attention!

(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)

The Free Dictionary

So I guess since you are too dumb to figure it out, I will have to spell it out for you. Cold Spots

... I've been bending over backwards/going out of my way for the last three hours to [explain it for you/make it clear for you]

bend over backwards and fall all over oneself (to do something) (for someone)

Fig. to work very hard to accomplish something for someone; to go out of one's way (to do something) (for someone). He will bend over backwards to help you. I bent over backwards for you, and you showed no thanks!

(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)