Go to the seneschal in a castle or town you already own and ask for "Diplomacy", then tell him you want to assign a fief to a lord. Choose the fief to assign, then choose yourself. You'll take a diplomacy hit with your lords, especially the one who loses the fief.
You must own at least one castle or city so you can talk to the seneschal.
The best way to gain territory is conquering. You get "friend points" for giving fiefs away, but you don't take a penalty for conquering territory, at least not if you're at war, so you might as well conquer and gift.
If you want towns for yourself, like those with Knighthood Orders already built, just conquer them and don't give them to anybody. To the victor go the spoils.