February’s edition of Gardener’s World points out that house sparrows have declined by 70% since 1977. Give them a boost by putting nest boxes in your garden; birds start to nest in late February in the UK so the best time to put these up is from now until mid February.
Don't know what sort of box to buy? The British Trust for Ornithology has some useful guidance about what to look for and where to site your box starting here: https://www.bto.org/about-birds/nnbw/buy-a-box. The direction of the entrance hole is not all that important as long as there’s a clear flight path to it. But it should be sheltered from the prevailing wind, rain and strong sunlight so in the UK anything from North to South East is probably best.
You'll notice they advise against siting boxes of the same type too close to each other to avoid aggressive behaviour between birds. Sparrows are an exception in this respect, they like to nest in colonies so a single next probably won't attract them. The RSPB sell a "sparrow terrace" consisting of 3 nest boxes joined to each other: https://shopping.rspb.org.uk/garden-bird-nest-boxes/rspb-sparrow-terrace-nest-box.html.
Whatever type of box you use, it's important to clean it out at the end of the breeding season. Bird Protection Law permits the cleaning out of nests between 1 September and 31 January, but if you're going for the earlier of these dates check first because birds may continue nesting until the end of September. Any dead eggs must be destroyed promptly and cannot be kept or sold.
I've just cleaned out and replaced my own nest boxes, here's what I did.
1. took the box down and opened it. This is a very common design, there is a bent piece of metal in the side which is actually the top of a screw. Once unscrewed the side of the bird box swivels to enable you to clean it.
2. removed any old material and scrubbed the box inside and out using an environmentally friendly washing up liquid. You can see the difference this made in the photo below, the dark side has not yet been washed.
3. Sterlised the box using boiling water.
4. Dried the box out very thoroughly by leaving it open over a radiator for several hours.
4. Mounted the box on a suitable tree, the wire is to help stabilise it as birds do not like boxes which move around.