Get a couple of pans of water on the boil, ideally one large, one small. The big one for the pasta, the small one for the shrimp. Since you will be straining both, you want to toss in loads of salt - think sea water.
Finely chop the garlic and parsley. Rinse the lemon and grate the rind, adjusting your grip regularly to avoid the bitter white pith immediately beneath the thin rind. This wants to be very fine so, depending on your grater, you may need to work over it with a knife.
Cook the pasta al dente. The trick to cooking pasta to the bite is not to multi-task beyond your ability while it's cooking, to prepare a fork, colander, and everything you need to strain the pasta as soon as it is ready, and to scoop it up and bite it regularly. Before you strain it, reserve a little more than a tablespoon of the water.
Now it's time for the shrimp. Wait the pasta out if you're less confident in the kitchen - you'll be heating it all up again in a moment. Either way, put the shrimp into the boiling water of the remaining pan and cook for three minutes. Meanwhile get a frying pan on the heat and, once you've read ahead through the next step, heat three tablespoons of olive oil, and add the garlic. You want to flavour the oil, even get a little colour, but not burn the garlic.
Once the garlic's good to go and without turning down the heat, add the strained spaghetti to the pan. Mix in the lemon rind, the juice of half a lemon, and chilli. Add a little of the retained cooking water so it doesn't dry out. Turn off the heat, mix in the parsley and season with salt and pepper.
Plate up with a nest of spaghetti topped off with the shrimp. Drizzle a little olive oil over the top and serve.