How to express this concept in English?

02:27 Dec 3, 2018 

"Every second [thing]" is not such a bad phrase for your general question, perhaps. The difficulty with your specific question is that you ask about doing first all of the odd, and then all of the even, so ultimately all are done.

If it were only necessary to do one half of a clearly ordered set, then you could say "Start with item 'A', and then every second one from there", or similar. Conversely, to include the other half it would simply become "Start with item 'B', and then every second one from there". Of course, 'A' and 'B' are just placeholders above, intending to indicate that numbering of the full set is unnecessary (if clearly ordered), but identifying the starting point (e.g. "Anna", "First Street", "Size 6") is necessary to avoid misunderstanding.

17:46 Nov 28, 2018 

Assuming that an action A is to be performed on n numbered objects, one could say: 'Perform A on odd-numbered objects, in order, followed by even-numbered objects, also in order.' For brevity, one might omit 'in order' and 'also in order' because that would be generally understood.

This is essentially the answer given by Hank Sanderson.