European Commission has created a huge portal, full of information for those interested in participating to the Horizon 2020 programme.
This Participant Portal as it is called, is the "participants' gateway and single entry point to interact with the Research programmes of the European Commission". It is definitely, the single most important collection of information for an organisation involved in a H2020 proposal.
In order to make the most out of it and of course submit a proposal, you first need to register here.
You can find all registration instructions in this link.
The portal offers a very detailed and "step by step" online manual which is available here.