The grid in the rain-on-grid (RoG) approach refers to the 2D mesh you create using the 2D Flow Area tool in HEC-RAS.

The unit hydrograph is the unit pulse response function of a linear hydrologic system. The unit hydropraph (originally named unit-graph) of a watershed is defined as a direct runoff hydrograph (DRH) resulting from 1 inch (usually taken as 1 cm is SI units) of excess rainfall generated uniformly over the drainage area at a constant rate for an effective duration. 

To import time-series data into HEC-HMS, the most common method is to create an HEC-DSS (Data Storage System) file using HEC-DSSVue. Here are the main steps:

In HEC-HMS, there are several basin modeling methods available, depending on the level of detail, available data and purpose of the simulation.

The basin model consists of multiple elements representing hydrologic features. These can be added by right-clicking on the Basin Model Map and selecting Add Element. The key elements include:

In HEC-HMS, when you click on GIS in the menu bar and then select Preprocessing Sinks, you perform a GIS operation to identify and handle sinks in the watershed model.Once performed this operation, it is not possible do undo it. Therefore, before doing it, be careful to have your DEM file correctly saved in your folder or create a copy of it.

https://spatialreference.org/This website allows to download the .prj file to set the coordinate system of your HEC-RAS and HEC-HMS files.

The first steps for the beginning of an HEC-HMS projects are: 1) Create a New Project, giving a Name, a Description and a folder location.File > New > Create New Project