Start off the battle by heading north and capturing the garrison right outside of your main camp. Now, all of the attention will be on you, so wait right outside of the garrison and take out all of the officers that show up. If Baima and Yanjin are not captured yet, assist in their takeover and return to the west side. If they have, stay put on the west side of the battlefield.
Note that after the above two have fallen into your hands, enemy units will likely come and try to take them away. One of these is Liu Bei and Zhang Fei, but they won't stick around for long because of Guan Yu returning to them and then leaving the battlefield. The officers defending the areas should be more than enough so that you won't need to back them up.
Xu You will end up defecting not too long after your plan has come to fruition, with him mentioning that the Wuchao Supply Depot should be set ablaze. This causes a fire attack unit to appear, which must be escorted to its target. Kill any officers in the way and once the fire attack succeeds, Zhang He will defect to your side, opening up the gate that leads into Guandu.
For now, fight your way through the castle until you get to the gate of the main room where Yuan Shao is located. Here you'll find Zhenji and upon defeating her, she will defect and join Wei. This will open the main room with Yuan Shao, so just defeat him to finally finish the battle.