Although you don't need to seal the cuts of small branches. But when it comes to a large branch cut we do need to seal it so that the bacteria does not infect the tree by entering through the cut, thus resulting in rotting. There are many types of cut pastes available in the market. But for those who are either unable to find one or can't afford, can make a paste of their own. What you can do is you can use plasticine, I bet everyone of you has played with it when you were kids ;). Mix it with some vegetable oil so that it stays soft and use it as a sealing agent.
Another option would be to use enamel paint also available in wooden color to seal the wound. Enamel paint has the property of being able to keep moisture from escaping and also keeps the bacteria out. Although it will wear off after sometime. You can either replace it or if you see that the wound has heal enough then you can leave it as it is. I myself haven't used plasticine but have been using enamel paint with good result for quite sometime.