So you'll have noticed a definite theme over the summer - short, quick, I've only got 10 minutes to myself makes! I've been solidly sticking to pimping and customising existing things rather than trying to whip things up from scratch. It's just as gratifying but takes way less time!
The key to this is, don't be afraid. Taking a pair of scissors to a top, sleeve, waist band can be intimidating, so maybe don't try it with your lovely Boden tee. However, a quick scout around a charity/thrift store, or everbodies favourite - Primani, and a bargain grabbed for small change, and you'll happily do anything to it! (I'd totally advocate the charity shop route though, you can actually find things made with really good quality material or much more interesting patterns than the plastic nonsense from cheap, high street stores.)
Want to do it yourself? I'd recommend working up your crochet piece before chopping bits off. That way you can pin it in place, and mark it up nicely before you actually make a cut. If you're going to do cuffs or sleeves, then use the first one that you cut off as a 'template' for the second - so lie it in place and mark where it come sup to on the sleeve. If you're going to do a yoke on a tee, then once you've marked it up, fold the tee in half and cut the neck line that way, so it's symmetrical. (Otherwise you can spend AGES nibbling bits off to try and make it even.) Also, make sure you give yourself a little bit of seam allowance.
Here's my fave thing I've been attacking this summer. My denim jacket has now almost become a crochet and stitching sampler, and the more I add to it, the more I love it.