US law and travel to Cuba for citizens of third countries

There are no direct flights from the US to Cuba. However there are direct flights from neighbouring countries. There are frequent flights from major Canadian airports, and from Mexico City and Cancun in Mexico (and probably other Mexican airports), as well as Jamaica and many Central American and Caribbean locations (not, obviously, US territories like Puerto Rico). To get to Cuba you buy a ticket to one of those places, and then another ticket to Cuba from there. You would need to use a non-US agent or website to buy the Cuba ticket. Many people do this, including US citizens.

There is nothing illegal about doing this if you are not a US citizen. Even the US doesn't believe it can regulate what non-US citizens do outside the US. You should be be careful that you do not do anything that could be considered trade with Cuba, such as bringing anything more than personal items purchased in Cuba to the US. It would probably be best not to advertize anything you bought in Cuba when you return to the US.

EDIT: Importing small quantities of Cuban-sourced goods is done all the time, even by US citizens. The Canadian border towns are full of stores advertising Cuban cigars, aimed at visiting Americans. Bringing personal items sourced in Cuba into the US is not itself illegal, only trade with Cuba. If you were bringing enough Cuban goods into the US that it might be considered trade, that would be a problem. Bringing stuff for your own use is not a problems.