Clarify simplified Chinese and traditional Chinese. Every Chinese learner should know that Chinese includes simplified Chinese and Traditional Chinese, and Chinese has 4 tones and mysterious strokes. Maybe you can speak Mandarin, but you can’t read Chinese. Mandarin is a form of spoken Chinese, while simplified Chinese and Traditional Chinese are written forms.
Have a right dictionary to search words. There are many choices for the dictionary, you can buy a Chinese dictionary, or use online dictionary, or install a free Chinese dictionary app which you can search words via pinyin, English and Chinese characters. Pay your attention on choosing dictionary, generally Chinese is simplified Chinese in Chinese to English dictionary or English to Chinese dictionary. If you would like to learn traditional Chinese, you should choose the dictionary supports traditional Chinese, or specialized traditional Chinese dictionary.
Have a good habit to read Chinese news, magazines or books.
Review all the words regularly you have learned (there are some rules to enhance the memory of words. For me, 127 rule is useful. I will view the words the 2nd day, and review them again the 7th day. You can try it.) It’s a bit difficult to write down every new word, especially the word has different meanings. Personally, I use a free Chinese learning app to create my own vocabulary list and add new words to my list. Then I can review all words in my list. The most convenient thing is that I can add notes to the words. Sometimes I would add a sentence with the word for better understanding or add another meaning of word.
Do training with writing. Writing the words can improve my memory for the word, and fortunately, the free Chinese learning app I use also have training section for me to practice writing (draw the strokes).
Speak with Chinese people or Chinese speakers. The most important step to improve vocabulary is to use it, because you will remember it more if you use it often. Never be shy to speak Chinese and never be afraid to make mistakes, instead you should seize every opportunity to practice it.