You'd need to get a brokerage account that allows you to trade sovereign bonds. (I think TD Ameritrade might do). And from there it should be a straight shot.
The only problem is that if you invest only a few hundred bucks, trading commissions (~$20 to buy and sell) eat up a big portion of your (potential) returns. For example, if you invest $200 and come out of your trade even (selling bonds back to the market for $200), you still pay $20 in trading commissions, so you'd be already down 10% on your trade. In other words, you'd need to be up 10% on your trade (which is very nice) in order to just compensate for commissions.
The only problem is that if you invest only a few hundred bucks, trading commissions (~$20 to buy and sell) eat up a big portion of your (potential) returns. For example, if you invest $200 and come out of your trade even (selling bonds back to the market for $200), you still pay $20 in trading commissions, so you'd be already down 10% on your trade. In other words, you'd need to be up 10% on your trade (which is very nice) in order to just compensate for commissions.