If I remeber right it blooms in spring so you'd probably be best off pruning just after bloom (although that'll mean you won't get fruits this year). As far as how to prune, there's nothing special about this vs any other shrub. Here's a general article on pruning for you http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/shrub... If you want it fuller, follow the instructions for heading back & pinching. This cultivar 'Compacta' naturally stays smaller and has a more compact growth habit, so it shouldn't need a lot of pruning. Maybe yours was labeled wrong and is actually one of the larger varieties?