How you apply modeling paste to create texture in a painting depends on what type of paste it is, how thick you wish it to be, and what support you're painting on. Start by reading the label; it usually says if there's a recommended maximum thickness for one layer and how long to wait to ensure a layer is dry.
If you paint too thick a layer, the top of it will have dried before the lower part, trapping moisture inside so it never cures or sets properly. For very thick texture, work in layers and be patient enough to let it dry thoroughly.
Using a rigid support (such as wood or board) rather than a flexible one (such as canvas or paper) means the painting won't flex or bend. This reduces the risk of dried modeling paste cracking. If you're only using a thin layer of texture paste, any flexing in the support is unlikely to be a problem. But if you've got a really thick layer, then it's going to be less flexible and if, for some reason the canvas or paper got knocked or jolted (say the corner of another canvas falls onto it or you dropped it), it may crack.