First you should be able to through check it.

Otherwise, just go to the transit hotel and check-in. The luggage will stay in lost and found in the secure area before the exit from customs. You then have two choices, go to the transfer desk within the secured area and request that they retag the bags for your LH flight. The alternative is pick up your waiting bag(s) and clear customs. Recheck at LH.

I leave bags until the morning on most of my visits to SIN.