Shop around! There are numerous companies offering joinery services but they all offer different aspects and you need to check they have the right skills for your project.
When you’ve narrowed down your search, go and talk to them, look at their facilities and ask for references or the chance to view previous work photos or testimonials.
Try and provide as concise a brief as possible, with an idea of materials and measurements to help with the quoting process. Discuss whether the works will require a site visit and survey
Once you’ve agreed the quote for the job, check their insurance certificates, this is your project and money afterall!
In order to make the whole process as painless as possible, agree, in advance, the estimated timescales of the job and any site conditions and rules which need to be considered, ie PPE requirements. Hopefully, at the end, you will end up with the expected quality product within your timescales and budget.