When designing a typeface there are a number of characters that I, as an English speaker, have never used, rarely seen and simply don’t understand. These characters fall in to a number of different groups, but all cause the same problem. I don’t know how to design them!
There are mathematical symbols.
Not being a mathematician, there are some of these I am familiar with, some I am not. But even the symbols I am familiar with can cause me problems. How do I design a plus and minus sign? I know what shape they should be, but what about alignment? Should the minus sign have the same vertical alignment as a hyphen? It often doesn’t but I don’t know why. When I get to symbols I don’t even understand the use of I am totally lost.
Some examples of mathematical symbols I am unsure of:
There are Non-English characters and diacritics. I don’t speak any languages that use any characters other than the standard 26 letters of the English alphabet. I have no idea how diacritics are used or should look. I also have no idea about characters like ß (sharp S), þ (Thorn) or ð (Eth).
Some examples of of characters I am unsure of:
My current approach is to find a font similar in weight and proportions to the font I am designing and using that as a base to design from, but I’m worried I may be getting things wrong.
Are there any official sources that describe how symbols and characters should be designed? And more generally—How do I approach designing characters I am unfamiliar with?
The following is very general (the last point will explain why), but mostly based on my experience with creating several special characters (most of languages I do not speak) for a blackletter font:
Search the Internet for design instructions from somebody from the community that uses a character, e.g., speakers of the languages that uses it. For letters, I try to maintain a list in this answer. Wikipedia is also not the worst source in this respect.
Take a look at existing typefaces that you expect to pay attention to properly implementing a character and try to identify possible design paradigms. Unfortunately this is easier said than done as even professional fonts may botch some special characters. Use your insights from point 1 to identify such bad typefaces.
In particular see whether you can find typefaces from type designers that should be familiar with the respective special characters, e.g., take a look at fonts from German type designers if you want to see proper ß designs.
The important thing here is to look for design paradigms rather than for fonts that are as similar as possible to your font.
Take a look at professional texts typeset in the respective languages. Be careful with texts from roughly 1980 to 2010 (the age of shitty computer typesetting and bad special-character support).
Try to get familiar with a character’s history and usage. For example, from the history of the ß you can learn that the left half originates from the letter ſ (long s) which in turn looks like the letter f without a bar. Thus the left half of your ß should almost always look like your f without the bar.
Use your brain and intuition. For example, the minus should look like the vertical bar of your plus. Also, if you consider some special character design to look ugly, out of place, too difficult to distinguish from other characters, etc., it probably isn’t good.
Ask a question on this site about that specific character. Stack Exchange is aspiring to accumlate such information and is very likely to last longer than privately hosted sites about a single language’s special characters. And even if Stack Exchange should go down one day, the CC licensing and available data dumps make it very likely that the information survives.
As a sidenote: Do not worry too much about mathematical typsetting. There are special fonts for this purpose. If you do not want to design such a font, it needs to be suited for everyday typesetting, nothing more.