Firstly, make sure the crank is turned back to 90 degrees before the TDC timing mark to ensure you don't damage anything while fitting the cams.
You will need to turn them to the TDC mark and then lock them in place. Do you have the locking tool used for cam belt replacement. If not, I would get one before proceeding further.
Often there are flats on the camshafts that you can place a spanner on in order to hold them in place. The valve spring tension will act on the lobes and cause them to try to move, so maybe get an assistant to slip the locking key in while you hold the two cams in place with a pair of spanners?
If you haven't removed the sprockets, it strikes me that you won't be able to correctly reinstall the camshaft seals?
Once you have the cams locked at the TDC position, then carefully turn the crankshaft up from the 90 degree before TDC position until the TDC mark lines up, then proceed to install the cam belt as you would when changing the belt.