Integration is a tricky topic to teach at A-Level (and beyond really) as so much comes down to experience. You only get good at Integration by doing lots of integrals and building up experience of different techniques.
Integration by substitution is very powerful but deciding on a substitution can be tricky and is often something students find hard. Because of this I wrote a small card sort where students first have to match an integral with an appropriate substitution and then do the integration.
If you want to use it the file is available here .