1. Explain it simply
A two-syllable adjective AB reduplicates to AABB, not ABAB.
高兴 becomes 高高兴兴, 干净 becomes 干干净净, 漂亮 becomes 漂漂亮亮.
The AABB form is vivid and descriptive: 高高兴兴地回家.
Add 地 before a verb, or use it after 得 to describe the result.
2. Show examples
孩子们高高兴兴地回家了。
Háizimen gāogāoxìngxìng de huíjiā le.
The children happily went home.
高兴 becomes 高高兴兴
她把房间打扫得干干净净。
Tā bǎ fángjiān dǎsǎo de gāngānjìngjìng.
She cleaned the room spotlessly clean.
干净 becomes 干干净净
大家安安静静地看书。
Dàjiā ānānjìngjìng de kànshū.
Everyone quietly reads.
AABB describes the manner
3. Share a tip
Two-syllable adjectives go AABB (干干净净), never ABAB (干净干净). Use 地 before a verb (高高兴兴地回家) or place it after 得 to show a vivid result.