A political party is not just  a group of experts who "offer" their ideas to the people (to voters)  during elections.   

A political party is a group of people who wants to develop/put in practice their ideas about public life . 

(If the candidates of your party are elected in the local (or any) elections, your party will be able to control the public offices in your council and assign  them to members of the party. Political parties are the only democratic means to control these posts.)

The main components of a political party are its members, the group of people that form it, who, usually, pay a (small) monthly fee, must assist to the meetings and activities of the party and are included in a book. 

Working with a large number of people is not easy. On the one hand, there must exist formative work: study, reading, discussion; on the other hand, there is a structural problem, there's the problem of how to structure a (large) number of people in a way that they can take decisions and these decisions do not become distorted.

When, three years ago, I thought of the possibility of creating a political party, I searched for information about these structures. 

I found that, at least in Europe, at the simplest level, the municipal level, (municipalities form territories, which also have a party organization, and territories form a nation, which has another)...at the municipal level, then, in each council, at the base of a political party, there's always a group of members, an "Assembly" (let's say, 600 persons), and an Executive Board (let's say, 6 persons) that is elected by the Assembly and is charged of representing the Assembly, directing the policies and checking the work of the public officers from the party (mayors, town councillors...) according to the instructions of the Assembly. 

Among the (three or four) statutes (political parties structures) I read (of the main Spanish parties) I liked the statutes of BNP (the Basque Nationalist Party) and chose them for my party, UDNG, for a number of reasons. Some of them were the following:

i) they ruled that public officers elected by BNP had to submit  an annual report on their activities to the Assembly, a report the Assembly had to approve, 

ii) they included  an internal judicial system (with courts, judges and trials), allowing the members of the  Assembly  to denounce the members of the Executive Board for any conduct that contravened the statutes, and 

iii) they established a categorical distinction between holding an "internal position", i.e. working within the party, and holding an "external position", i.e. working (as a member of the party) "outside the party", (as "public servant", mayor, town concillor, etc...), establishing that normally both positions were not compatible and a member of the party shouldn't hold an internal and an external position at the same time .   

As UDNG (copying the BNP) says in paragraph e) of Section 3) "General Principles": 

e) (UDNG) assumes the principles of separation of internal and external posts, of non-accumulation of power and of no confusion between the figure of the person charged of exercising control and the figure of the person who has to be controlled. 

iv) they recognized the sovereignty of the Assembly, ruling that  the "assemblies competent for the election of someone for an internal or external post are also entitled  to revoke this election in any moment". 

It's strange that, being so important, the statutes (the organizative structures) of political parties are not documents well known and discussed, and  its comparison is not a part of the election campaign, because they seem much more than "internal documents", they constitute the way the party manages the people that form it (or will form it). It is also strange that  intellectuals, sociologists, structuralists, human resource specialists and so on do not work on the design of better structures.  

A final observation: in UDNG, when in a town there are many members, when the Assembly is too big and its size makes it difficult working, the original Assembly can be divided into as many Assemblies/Executive Boards (two, four, eight) as seems necessary, and each of these units (Assembly/Board) elects two or three members to represent it in a new Coordination Board, called "Interjuntas"("Interboards"), which is created to coordinate the proposals of the Assemblies. 

It seems that the BNP (the Basque Nationalist Party) (and UDNG) are the only parties in Spain (in Europe?) that have taken into account this possibility. How do other parties manage in cities with a population of millions?