。
This was a pretty well-thought-out plan.
Literal
This [topic-は] quite well practiced-ていた plan [copula-だった] [informative-ね].
練る ('to knead, refine, polish, work out') is used metaphorically for refining ideas or plans. 練られる in the passive: 'was refined/polished.' ていた is the past continuous giving a 'had been worked out' reading. かなり ('quite, considerably') is an adverb of degree.