When you are dealing with connecting a multi-winding transformer you can connect winding in series or in parallel.
If you don’t connect them properly you can get a dead short or what I commonly call Kablazalflam.
It is very important that when you are dealing with the primary connection that you are paying attention to the direction of current across each winding. If the current is heading in the same direction across each winding that is good. If not then you have an issue.
When you are looking at the secondary side, it is important that you observe the instantaneous polarity. Remember that the line side determines the primary polarity and the primary polarity determines the secondary polarity.
To see what I am talking about, take a look at the video