Your question is super important. I've worked with numerous contractors on projects.
You'll want to grasp two things very well: 1.) Management 101: "Trust, but Verify." If you hire someone, you need to verify that they did what they said they would do, at all stages, not just at the end. You need to check their work as they go. Assuming they provided referrals and come with a solid reputation, you should extend a sense of trust but make it clear that you'll verify their work and that payment is dependent on your approval according to what you agreed. Note that this extends to verifying important details, like whether they have liability and worker's comp insurance in place. I had a contractor forward me his insurance policy, and I called the insurer, only to find out it wasn't correct. Your role is a manager and good managers have to verify everyone and everything on a project. 2.) You want to be as detailed as possible in any contractor agreement about the work to be performed. That's why we have architectural and engineering specifications, to show precise measurements, along with detail drawings showing specific elements and steps required for things like exterior walls. You want to be as detailed and descriptive as you can reasonably afford to be, for the type and size of job you are doing. A light renovation requires less specificity than a new construction build, and you'll have to gauge that accordingly, but you should err on the side of being more specific rather than less.
Lastly, regarding materials purchases, you should NEVER let a contractor choose, purchase or pay for any finish materials or important elements like appliances, sinks, toilets, bathtubs, counters, cabinets, trim, doors, windows, or even things like paint and stain or varnish. Assume that contractors will ALWAYS choose the cheapest (and ugliest) material for a task. Trust me, I've seen their choices myself over the years, and you don't want to get into an argument about an ugly tiny useless bathroom sink after the contractor bought and installed it. It's really your fault if a contractor is given free reign on a project to make design decisions. They are called contractors and not designers for a reason, (because they aren't paid or incentivized to have any design sense, just efficiency in execution) and they have a financial disincentive to choose quality, expensive, labor-intensive, or unique materials. So even with paint, for example, you want to specify up front what brand and grade they must purchase, if you are having them purchase it.
It's best if you purchase critical materials yourself, or specify exactly the material the contractor needs to buy and then be sure to verify he buys the right materials.
