How do add new spouse if they have children from another marriage
The children from another marriage really has nothing to do with the question of how to add a spouse. You add a spouse by adding a spouse.
If the new spouse with children from another marriage is already in your database, make sure you are on the main People tab with the person highlighted for whom you wish to add a spouse. Click the + icon in the upper right corner and choose the Add Spouse option. An Add Spouse window will open with options to Add New Person or Select Existing Person. So choose the Select Existing Person option. The new spouse will be added, but the children from a previous marriage will not become a part of the new couple’s family.
If the new spouse with children from another marriage is not already in your database, follow the same procedure except choose the Add New Person option. Since the new spouse was not already in your database, the new spouse’s children from another marriage are probably not already in your database and you will need to add them. I suspect that the thrust of your question would be how to add the children so that that they show up from the other marriage and not from the new marriage. To do that, the easiest way is first to add the previous spouse for the newly added spouse which will make a new family in your database. Then add the children from the previous marriage to that family.