The hole of the shelter decides which birds are going to use it, so if you want small birds like Sparrows or tits to use shelter, use the shelters which have a opening of 32mm.
This is important, because if the hole is bigger the birds like crow and myna can destroy the eggs.
If there are many parakeets in your location and you want them to stay in your nest boxes you can opt for the shelters with opening of 50 mm i.e. 5 cm.
The shelter should be placed at such a position that birds feel safe and comfortable. In urban settings the shelters for sparrows can be placed near the meter boxes or in the area above the staircase of your building.
Or Place the shelters where you have earlier seen the birds nesting. Ensure the nest boxes are sheltered from direct rain, wind and strong sunlight.
Also do not place the shelter and feeder very near to each other as the birds attracting the feeders can disturb the nesting couple.
The care to be taken is mentioned in this link : Shelter/Feeder Care