While writing description you can decide your budget for project as hourly or fixed rate. There are some points which need to understand about fixed rates.
Freelancers submit their proposal bid for the entire project price at a time. They can negotiate about price in interviews later. They can set milestones if project seems to be complex and time taking.
If projects seems of be large enough you can split into multiple parts and set prices according to parts milestones. This is fixed price process, bid a project completed your task and get amount after successfully delivered. The amount will be saved in escrow, you will get after delivering the project.
For small project, complete requirements get your amount and task done
For large projects preferable to set milestones and split amount according to that.