For your chosen period and style, read up about it at Bricks & Brass, look for a book or two, and visit a museum or period home open to the public.
Understand the styles of fireplace typical for your period and style. For central heating you will need radiators that are discrete in style or can be painted or disguised.
Electric lighting began to appear only in the Edwardian period; gas lights were the norm before this, with oil lamps and candles even earlier. This is another area where compromises will be needed for most period home owners.
Research the paint colours, wallpaper and fabrics. Would your home have used wallpaper or just paint, and in which rooms?
Look for motifs typical of your period; for example florals in the Arts and Crafts, and very stylised shapes with Art Nouveau.
What flooring is appropriate? Was bare wood, brick or stone the usual material or would it have been covered by carpet or rugs, or even both? Perhaps carpet and rugs would have been reserved for the drawing and dining rooms.
Tastes in carpeting have shifted dramatically over the past two centuries; the latest fashion for a central carpet or large rug revealing wooden floors at the edge was last popular from the late 19th century when new discoveries about hygiene meant that carpeting had to be taken up regularly so it could be beaten clean outside.
Assuming you have original or appropriate doors, architraves and skirting, you need to understand the styles of door furniture, and the decorative treatments used for the door and other woodwork.
The final step is to create the designs for each room.