Here's the laptop charger, the Sugru, the cable ties, and a pair of snips for cutting the cable ties.
Next, I put a plastic tie at both ends of the cable (because I'm giving the Sugru treatment to both the connector end and the power brick end). It doesn't have to be super-tight, but you do need to be able to snip the tail off the cable tie as close to the lock as possible.
The purpose of the cable tie is to give the Sugru something to grip onto. It does work without this, but I've found that this gives a more durable fix.
Here's the Sugru. I'm using black because that's what I had. It does make a bit of a mess, so if you want to be tidy I suggest using white.
Now you just start molding it over the cable and the existing strain relief. If you're thinking of doing this on a new power cord, I actually suggest you wait a few months because you'll find that the cable usually takes on a particular bend or twist, and then you can mold the Sugru to follow these bends. This, believe it or not, makes the fix stronger.
And yes, I know my molding is messy. I never was any good at crafts.
See how the Surgu molding at the connector end has a bend in it? I'm following what seems to be the way the cable wants to bend.