One of the charming features of Chinese is reduplication (repeating, or doubling up) of certain words and characters, including adjectives! Reduplication makes the descriptive feeling of the adjective stronger.

One-Syllable Adjectives (AA)

If an adjective is only one syllable (one character), then reduplicating it is a no-brainer. In most Chinese textbooks and grammar books, this is known as the "AA" pattern.
Adj + Adj + 的 (+ Noun)

Another way of putting that (and we'll be using this form more extensively below) is:
A A + 的 (+ Noun)

Note that this pattern is also identical to single-syllable reduplication of verbs.

Māmɑdeshǒu nuǎnnuǎnde.
妈妈的手 暖暖的。
Mother's hands are warm.

Nǐde liǎn hónɡhónɡde.
你的 脸 红红的。
Your face is red.

Guǒzhī tiántiánde.
果汁 甜甜的。
Fruit juice is sweet.

Two-Syllable Adjectives (AABB)

If the adjective has more than one character, then you should repeat each character individually (rather than the whole word). This is known as the "AABB" pattern.
In the structure below, the original two-character adjective (such as 高兴, 漂亮) is "AB," where the first character of the adjective is represented by "A," and the second by "B."
A A B B + 的 (+ Noun)

Not all two-syllable adjectives are used in AABB form, but here are some common examples of AB adjectives represented in AABB form.

ɡāoxìnɡ → ɡāoɡāoxìnɡxìnɡ
高兴 → 高高兴兴

kāixīn → kāikāixīnxīn
开心 → 开开心心

rènɑo → rèrènɑonào
热闹 → 热热闹闹

ɡānjìnɡ → ɡānɡānjìnɡjìnɡ
干净 → 干干净净

Using Reduplicated Adjectives as Adverbs (AABB)

When adjectives are used as adverbs to modify verbs, you can reduplicate the adjective using the AABB pattern.
A A B B + 地 + Verb