The Commission publishes a proposal.
This is sent to the European Parliament and the Council of Ministers.
A lead (and several other) Committee(s), in this case the Environment Committee, in the European Parliament will start working on the proposal in the European Parliament under the leadership of a rapporteur. The rapporteur will publish a 'report' that will suggest a raft of amendments to the Commission's legislative proposal. The amendments will be those proposed by the rapporteur, but also by other MEPs in the Environment Committee (and often also compromise proposals agreed to by a group of MEPs across party lines).
This report is voted on in the Environment Committee and is then ready to be voted on in the plenary sitting of the European Parliament.
At the same time the Council of Ministers will be trying to negotiate a common position too.
If the Environment Committee's vote is very based on very strong majorities, it is possible that the European Parliament, the Council of Ministers and the European Commission start a process called 'trilogue'. This is an informal process outside of the formal legislative process in which the three institutions try to find an agreed text that they all support.
If the vote is not clear, there will not be a trilogue yet.
From the Committee vote, the Commission's proposal and the suggested amendments from the Environment Committee (and other Committee's, but less important; and some other amendments proposed by groups of MEPS, also less important) are voted on in the 1st hearing in the European Parliament - in plenary sitting (i.e. of the whole Parliament).
If there has been agreement in trilogue, the agreed trilogue version is voted on in plenary, which is almost always voted through.
If there has been no trilogue, then it is likely to happen after this 1st hearing.
The official legislative process continues too though, which consists of the Parliament's text being sent to the Council of Ministers, who can either accept it, or suggest changes or reject it.
If the Council suggests changes, then the instrument goes through a second reading in the European Parliament. Much of this will be happening in trilogue.
The rest of the 2nd hearing process is much the same as the first time round.
If the European Parliament and the Council of Ministers cannot agree a text on 2nd reading, then there is something called a conciliation procedure and a 3rd reading. This does not happen very often.
Diagram above (from the above codecision report) contains Parliamentary information licensed under the Open Parliament Licence v1.0