This activity provided through the OER commons from Math Open Source is a great resource to help student develop a definition of similarity.  http://www.oercommons.org/courses/similar-polygons-and-triangles-defined/view

This quizlet by Gabe Angelly can help students practice corresponding parts of similar polygons and triangles.  I would suggest doing this as a quizlet live activity.  https://quizlet.com/350526811/cpctc-flash-cards/

CK12 has an adaptive practice on similar polygons and scale factor which provides students practice with feedback.  https://www.ck12.org/c/geometry/similar-polygons-and-scale-factors/asmtpractice/Similar-Polygons-and-Scale-Factors-Practice/?collectionCreatorID=3&conceptCollectionHandle=geometry-%3A%3A-similar-polygons-and-scale-factors&collectionHandle=geometry