There are many ways in which one can improve the appearance of internal walls. I will briefly explain three different methods.
The simplest method is to plaster over the existing wall. This may be problematic as certain surfaces may not accept fresh plaster. The surface must always be secure prior to scimming the wall. This method will leave a flat finish but not a levelled wall. This depends on whether the wall was level in the first place.
A box screed system will leave your walls level but a lot more involvement and expense is required. A floating coat is needed before finishing plaster. Using this process, the walls will be straight and solid.
However, dry lining is now the most common way of achieving flat and levelled walls. Sheets of plaster board are secured to the background surface which will leave an air gap between the old surface and the new. This process will increase the warmth in the house as the plasterboard will act as an insulator, whereas solid plastered walls can leave the room cold. (You may have experienced the cold in old houses.) Dry lining is a relatively quick process and will dry out fairly quickly, enabling one to continue with the decoration works – which in turn means the room will be ready for use in a short time.