I believe there is an example of construction the transactionInput where the nonce is one of the property. The default way AFAIK is to get the # of transaction submitted for the sender. This would sort of 'order' multiple transaction submitted. Though you can also keep a counter of your own.
one thing I don't quite understand is this nonce thing, I would think it is a random#(thus nonce) but it actually has the potential of controlling the execution order ? As otherwise, why not generate just a guid