Mark this is a very long process, but here you are some brief keys:
Make a REAL budget for all your expenses in the event. Then decide ¨which percentage¨ of the final number your sponsors should fund (in Spain is usually the 50%). Be aware that nobody is going to cover the 100% of the initiative, you should search for other sources if financially you can not afford it (institutional, crowdfunding, partners,…etc).
With that number already defined for the sponsors, consider the idea of building a ¨one main sponsor¨ policy or, in the other hand, adding collaborators to the promotional strategy (second level sponsors) . With that, you will have the ¨final number¨ to charge the sponsors.
Finally, take in consideration that sponsors always search for ¨repercussion¨, mainly through the mass media. You should prepare a document for this field including the general balance for your first edition´s experience and other similar events if you had the experience before. This is SUPER IMPORTANT for the sponsors, they distrust when there are no ¨data¨ to justify an investment. Collect all the benefits of the action that you offer to the sponsor for the money they have to put, and make them see that your proposal is much cheaper than an advertising campaign with similar repercussions (this is important too).
With all this, they will analyze the situation, will compare everything with other parallel proposals, and with the final balance, they will let you know about their final decision.
3+1) One interesting ¨hook¨ to catch the attention of the potential sponsors, specially in firsts editions like this one (I guess it is your second edition, isn´t it?), is giving them the chance to sell their products ¨LIVE¨ in the event. If you are expecting ± 7,000 people attendance or more (that is amazing for a first experience, congrats), I guess you have here a big strength to deal with the negotiation. They will pay attention to you.
Good luck with everything!. Regards.