Welcome to hetalia_tweets, where you ask not what your country can tweet for you - you ask what you can tweet for your country.
1. Check the list of Twitter circles (listed below!).
2. Find one that has an open slot for your character of choice.
3. Post to the entry corresponding with the circle.
4. Make account once accepted.
5. Follow everyone on that list.
For example, if you want to play Spain but see that the first two lists already show Spain as taken, just go to the third list and join up there! As each circle gets sufficiently self-sustaining, I'll add more.
♥ Regarding Replacements
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