How do I correctly borrow chords from one key to another?

(1) The term "borrowed chords" usually refers to chords from another scale with the same keynote. Most commonly this is done in a major key, when chords are taken from the parallel minor. So, in C major, you could "borrow" Eb, Ab, Bb and Fm from C minor.
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How to borrow chords from other keys.

1. How do you transition from one music key to another?
2. How do you get chords from keys?
3. Why do borrowed chords work?

How do you transition from one music key to another?

The smoothest way to modulate from one key to another is to use a pivot chord. A pivot chord is a chord that both keys share in common. For example C major and G major share four chords in common: C, Em, G, and Am. Any one of these chords can be used to transition smoothly from C major to G major.

How do you get chords from keys?

For every note in the scale, there is a chord that starts on that note. The way to figure out what chords are in a key is to look at each note in the key and use a pattern to figure out each chord. The first note, C, gives us the C Major chord. The second note, D, gives us the D minor chord.

Why do borrowed chords work?

Borrowed chords are typically used as "color chords", providing harmonic variety through contrasting scale forms, which are major scales and the three forms of minor scales. Chords may also be borrowed from other parallel modes besides the major and minor mode, for example D Dorian with D major.