If you'd like to give the insects in your garden a bit of a helping hand, an insect hotel is a fairly simple project that might make a real difference. It's nothing fancy, but here's roughly how you could go about it.
To make an insect hotel, you'll want to get hold of a few small logs (picture: small logs), some bark, and other bio-material — leaves, hay, and straw work reasonably well. You'll want to drill some holes in the small logs, perhaps around 10-15 cm deep, with widths somewhere between 3-12 mm. It's probably best to drill most of the holes at about 6-7 mm wide, roughly 10 cm deep. It might be a good idea to avoid pressure-treated wood for the holes themselves. The house itself, on the other hand, should be fine made from pressure-treated wood.