Measure the gap between the valve guides, which should be no more than 0.2 mm.
If the gap is larger, remove the oil seal and use a drift to press out the guide sleeve, acting from the combustion chamber to the valve control mechanism.
Press the new guide bushing into the cylinder head body using tool 49 0223 160 D.
Install a new oil seal.
Check the concentricity of the new guide and valve seat, grind the appropriate seat if necessary.
Lap in the valve seats using the new bushing as a pivot.