The great thing about Extension 1 Math is that you get given the substitution (at higher levels you need to make the decision, and it can be more time consuming than the math itself).
Example: Use the substitution below to evaluate the integral below:
Make your working for these questions clear; I like to do all my differentiating outside of the integral.
We then substitute and evaluate. If you read one sentence in this guide, read this one... Remember to substitute the changes to the upper and lower limits of the integral! So many people forget, don't let it be you :)
The other thing to remember is that if you are doing an indefinite integral, make sure you put the answer back in terms of x. Definite integrals are a bit easier in the sense that you can forget about x once it is out of the integrand.