Comparing forms of humble self reference
If you want to use 謙稱 with accuracy, you'll have to learn how to use 尊稱 too! You can't have the yin with the yang. Learning all this is a big job, with not much payoff, except a reputation for nice writing. 

If you're determined to try, be wary of all internet stuff; I found one sort of okay thing from Hong Kong's Civil Service Bureau here 

There are also books on 應用文 in Taiwan; these focus on various types of 'applied writing': letters, invitations, minutes, notices, etc. They include a lot you probably don't want to mess with.  There are a few old handbooks of letter writing I've seen around, look for titles like '尺牘大全'. I know of ONE book in English: "Business and correspondence Chinese : an introduction", by Liang, Haft, and Mulder. Good luck on finding it. There is also a technical study by 曹湘洪: 当代汉语私人书信称谓, 起始及结束语社会语言学硏究 alas, for sociolinguists, not epistolarians. 

Generally, most modern writing that tries to use 謙稱/尊稱 winds up sounding like an American actor in a British costume period piece. If you just want to have fun, go ahead, just be aware you might wind up sounding even funnier than you want to.  

I also feel that in this case, Wikipedia is not a reliable source. The article on Chinese Honorifics mushes together generic terms with highly specific terms, all taken from dynasties randomly scattered across Chinese history, throws up etymologies without any sources, and ends by explaining that to use 老子, meaning "I, your dad" is a way of "referring to oneself as superior"; hence, "when used towards a person less well known or on formal occasions, both terms are considered to be incredibly rude." Yep, Chinese are pretty subtle people...