A. Generally a top dressing will be a combination of loam, sand and peat, not multi-purpose compost, which can contain weed seeds. The mix is dependent on soil type: for a loamy soil 3 parts sand to 3 parts loam to 1 part peat. The sand needs to be lime free and suitable for top dressing, no sea sand and grain size is important so seek advice at the garden centre.
The top dressing should be dry to get an even mix. For heavy clay soils increase the amount of sand and reduce the amount of loam. For sandy soils reduce the amount of sand. Top dressing is quite a technical subject so it's worth researching in depth for the best results.