Step 1: Create the system

We are going to make a one layer Li (001) slab with a 2x2 unit cell, assuming familiarity with the build tools

From the structure tool select 'Cubic' and 'bcc'
Set 'Element' to Li
Set the lattice parameter to 3.49
Press 'Apply' and 'Close'
Invoke the Slice tool
Set the Miller indices to 001, select 'Cartesian', and enter 1 layer.
Press 'OK'
From the 'Edit' menu choose 'Define Super Cell...'
Select the preset '2x2' and press 'OK'

Your screen should look like this (after selecting the 'Coordinates' panel)

Add with the mouse two hydrogen atoms anywhere in the screen
Select the 'Normal' edit mode
Set in the table the coordinates of the first hydrogen atom to (0, -0.5, 2)
Set the second H atom coordinates to (0, 0.5, 2)

The final geometry looks like this