It is impossible to give any in depth guidance on the conservation of gilt leather on a site such as this. Gilt leather is unusual, and does not respond to treatment in the same way as some other objects. However, there are a few pointers and warnings which may assist. (mostly warnings!) For example, it is much more difficult to clean than many oil paintings.
Always seek advice from a specialist if you are not sure.
Cleaning. The yellow varnish on gilt leather is usually very readily soluble in polar solvents. Any attempt to clean it using solvents such as acetone and alcohol is likely to be disastrous. The solvent will pass through the paint and into the underlying varnish and the paint layer will become detached as the varnish dissolves. Unfortunately non-polar solvents are usually poor cleaners. As a result, in depth cleaning is generally impossible. Specialist help from somebody with experience of gilt leather is essential. Careful dusting with a soft brush is usually all that can be done by the non-specialist.
Oils and dressings. The temptation to apply oils and dressings to the leather is high, but it should never be done. The oils and fats have a strong tendency to migrate into the varnish and paint layers. This can turn the decorative surfaces brown, and in extreme cases, can make the paint and varnish so soft that they can run down the leather when it is vertical. Removing excess oil and fat is extremely difficult or impossible.
Humidification. Gilt leather is often distorted, and humidification is often needed to reduce this. As with all leather, heat and moisture must never be used as it can lead to catastrophic, irreversible shrinkage. The "shrinkage temperature" at which the structure will collapse in the presence of moisture can be as low as 30 - 40 degrees centigrade. As a result steam must never be used. (Similarly, heat must never be used to dry water based adhesives). Cold humidification using ultrasonic humidifiers, or semi-permeable membranes should be used, but only after testing to check for adverse effects.
Finishing. It is very easy to apply a varnish to brighten up gilt leather. This has often been done in the past, and the varnishes are now discoloured. They are very difficult, if not impossible, to remove, as the solvent also dissolves the underlying original yellow varnish. In theory, some modern varnishes should remain easily soluble and could be removed, but we do not really know what will happen in 100 years time. Adding varnish now could be making terrible problems for the future and is usually best avoided.
Similarly, the addition of over paint to improve the appearance is seldom successful. Again, in cases where this has been done in the past, removal is very difficult. This is not the same as in-painting just the areas of damage with a material such as artists' quality water colours, this can improve the overall appearance and is unlikely to cause long term problems.