It sounds as if you are just starting your career, so congratulations on your new profession.
There are many ways that designers charge for their time. Hourly is becoming a standard for most professionals. The average rates are $75 - 150 per hour. Some charge that rate portal to portal (door to door for any work they do).
An initial consultation is usually a "get to know each other" opportunity to decide if you are a good fit. Be very cautious about offering too much free advice in the first session.
It is wise to have a contract, and some of us require a retainer for our services.
Some designers charge lower rates for the administration part of the jobs.
Billing monthly or even every two weeks (we do this) is important to keep your cash flow going.
Ironically, the ASID survey showed that almost 80% of all consumers preferred a fixed fee basis. This is a hard one to figure out when you start out...and even if you are experienced.
Most designers charge a mark-up on products and it seems that 25 - 35% is the typical mark-up. Draperies can be higher.
These days, you will find that clients will shop your prices and may even try to get their sales tax license to purchase their own products at the design center. If so, you will have to decide how to handle this. We actually interviewed a few designers (we also do this) that let their clients do their own purchasing. Since administration and expediting take up such a large percentage of time billing, and have the most headaches, we don't mind them doing this because they really find out how hard the process is. If you have disputes on bills, it is about how much time it takes to do this part of the work.
The most important thing to remember is to be very clear with your clients about how you bill. Be sure to set realistic expectations about what your bills might run for a project and make sure there aren't any surprises.
A final word of advice, for now, is to be sure you work with the right clients. If you are just starting out and need to pay the bills, it can be tempting to take clients that aren't a good fit. I would highly recommend that you be discerning and careful. Difficult clients take time away from good clients, and there are many good ones out there.
I hope that helps.
Gail Doby, ASID
Design Success University