Generally speaking, the first condition to doing business as a foreigner in San Miguel de Allende is to obtain or make sure you will be able to obtain the proper approval and visa from immigration. Unlike the US or Canada, where after getting a visa to work you can do any business activity you choose, in Mexico, every specific economic activity will need approval.
The easiest activities for approval are artistic in nature, namely, artists who are making and seeking permission to sell their art. Beyond permission to make and sell your own art, permission gets a little more difficult depending on what you want to do. The general idea is that Mexico does not want to give approval to foreigners to do work that could easily be done by its own citizens. Additionally, if your field requires a license, such as in medicine, or architecture, you will be required to submit all of your certifications and you may be required to be approved first by the appropriate licensing agency in the state.
Working for an existing company may be much easier than getting approval to work on your own. In this case the potential company where you will be employed will write a letter on your behalf and “sponsor” you. The sponsorship gives you permission to work only for the company.
Although many people living in San Miguel work a US job via the internet, have only a retirement visa, and no other approval, information regarding this type of situation is conflicting. However, I have never heard of anyone being questioned regarding their work from another country and have only seen immigration issues arise for those charging for a product or service within Mexico.