It would help if you said what make/model head you had. However, most are plumbed much the same...so if you don't know what hoses are what, it is usually pretty simple to trace them.
The head's water intake hose is usually a 3/4" hose and goes to a seacock and through hull. The head's output hose is 1.5" in diameter, and either goes to the holding tank, which is my preferred setup, or to a diverter valve, and then either to the seacock and through-hull or to the holding tank.
Since you don't have a head installed currently, IMHO, you should look at replacing the hose as part of your installation, since permeated hoses and calcification inside hoses are a common issue. The best hose for the install is either Trident 101/102 or Sealand OdorSafe for the waste hoses, and Trident 148 for the intake and vent lines.
Use stainless steel hose clamps and double clamp the below the water line hoses. Make sure the clamps are high quality, all stainless steel clamps, like the AWAB or ABA brand (which are the same make under different names).
The head itself will generally have four fittings.
First, there is an intake hose fitting that is 3/4" that gets connected to the seacock.
Then there is a water output to the bowl, which is 3/4", which is connected to the water input to the bowl—also 3/4". This line should be a high vented loop if the head is installed below the water line.
Finally, there is a waste output fitting from the pump which is the only 1.5" line.
The Holding Tank will generally have three fittings.
One will be a 5/8" vent line.
One will be a 1.5" fitting that is mounted high, in the case of side mounted fittings, or not have a pickup tube inside the tank in the case of top mounted tank fittings. This is the waste input fitting that should come from the head.
The other 1.5" fitting will generally be mounted low in the tank for a side mounted fitting or have a long pickup tube for a top mounted fitting. This is the pumpout hose fitting. Ideally, it should go to a diverter valve and the valve outputs should go either up to a deck pumpout fitting or to a macerator/diaphragm pump and then to the seacock and discharge through-hull.
I hope this helps.
Sorry, Marina Del Rey.
I am looking for an owner manual so I can figure out the in / out hoses, I can maybe try it. Unfortunately the original head was all ready taken out so I don't have enough references.