Passage of a Bill
An Act of Parliament starts out as an idea for a new law. The ideas for new laws can come from parliament (primary laws) or department ministers (secondary laws). Wherever the idea for a new law comes from, it must be introduced to the Cabinet as a whole following approval and drafting by the relevant Cabinet Committee.
A green paper is, essentially, a consultation document. It sets the idea for a new law and the reasoning behind it. The Minister for the department who the potential law concerns is responsible for the green paper and will be the one who the debate and questions are aimed at.
For example, a proposal on road traffic laws might be of interest to the police (who have to enforce it), the legal profession (who may have to advocate for or against it), motoring organisations such as the AA and RAC (who uphold their members interests) and car manufacturers (who may need to review designs). All of these parties may wish to have their say on the proposal for the traffic law. In this case the minister may have to amend, re-think or re-draft the proposal, before moving to the white paper stage
A white paper is what comes after the green paper once it has been amended, re-thought or re-drafted as a result of the law commissions report. A white paper is the set of proposals for the bill which is passed to the relevant Cabinet committee and must be approved by the whole cabinet. Once it is approved it is handed to parliamentary draftsmen to be drafted into a Bill.
A Bill may start in either the House of Commons or the House of Lords (except finance bills which MUST start in the commons)