Pick up the title from the campaign map, drag it over the general. Then simply let it go. To remove it again drag an emissary over the general and in one year the titles will be stripped. The general's loyalty will drop somewhat however.
When assigning titles the general rule is to find those generals that have good acumen. Though for rebellious or recently conquered provinces you will also be looking for a general with high dread. Piety is also a factor. If the province's religion matches your faction's, then assign a high piety governor. If it doesn't then assign a low piety one. This will counter any unhappiness caused by a governor of a different religion.
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