We recommend putting enough straw, hay or other suitable bedding to cover the floor with a thick layer.
Hedgehogs will start to move things around inside the house and pile bedding up in the area they want.
Leave some bedding outside for them if the need it
You can leave the material to build bedding outside the house and watch your guests move what they want in. This can be Hay or just leaves from around the garden.
More info on using your Hedgehog House can be viewed here: https://homeandroost.co.uk/blog/hedgehog-house-guide/