You may create single bonds or equivalent bonds (same length, same partners) by selecting from the menu [Bonds]. Then you have to click at the two atoms to be connected.
In case of equivalent bonds:
If the actual lengths vary and you want them to be created anyway (like in quartz) you may set a higher value for the allowed deviation in the bondlengths with [Bonds][set deviation]. The default deviation dl=0.01 .
For bond properties click here.