Hiring a theme developer - what info should I prepare?

Yes it will be good if you can provide PSD for every page. Every PSD should be layered properly so that images can be sliced easily and correctly.

Devender

Ideally you'd have a document outlining the needs of the site - what page types you have, any special display needs (e.g. slideshows, drop-down menus, anything requiring JavaScript), any special features like event calendars, image gallery, etc. To do a quote I don't think a full PSD is necessary. Just static PNG's or whatever is fine. The important thing is to be able to see all key page types.

About the process - personally I don't really work that way. It's always necessary to get some assets out of the PSD's, but must of what I do is working with CSS and Drupal templates. I also usually end up doing most of the site implementation, including creating content types, Views, and installing any modules required to implement the design (e.g. slideshow, menu drop-downs, etc.).

Not everyone uses PSD, I guess it depends on the themer. Other apps that can also count are Inkscape, GIMP, Fireworks, Illustrator...

The time when you would take a template and slice it up into table cells or absolutely positioned objects is long gone I think, at least I haven't stumbled upon examples of it. What you need from the graphics file is an easy way to slice up appropriate parts of the design (which are usually only background images, logo, maybe some content images) and export them into the most appropriate format (jpg, png, transparency), then they are assembled into a theme through php templates and CSS.

Whatever software is used, it's best if the themer has access to the original file with all the layers, meaning to use the same software as the designer. It's actually not necessary to have all the layers, the original design template might be reduced to only a handful of layers to make it quicker and easier for the themer to handle but still have enough "layering" preserved for easier slicing. But designers don't always know CSS enough to know how to make that kind of preparation. It also depends a lot on the themer use of CSS, if she uses CSS gradients and not images for modern browsers she will need gradient information... Maybe your themer and designer can collaborate in a way that the themer tells the designer which "slices" he needs and the designer exports them.

It's good to name the layers with meaningful names, don't just use object1, object2 etc so it's easy to navigate through the layer tree even to someone new to the file, but this is also beneficial to the original designer as well. Grouping layers into sets and subsets is also useful if it's done with care.

You also have to think about the behavior and other effects, for example what should the menu buttons look like on hover, or form input elements on focus. Usually those styles come as separate layers or layer sets.

Often some design elements are left out if the designer isn't familiar with Drupal, or for some other reasons, this often includes Drupal tabs (links View, Edit etc you see when logged in if you have enough permissions, but also on /user page for anonymous users), form elements (input fields and submit buttons), favicon. I usually extrapolate from the design to style those if they are missing.

Provide a different design template for every structural part that will look different, you don't have to repeat templates for every content type if they all follow the same design. Usually you have the default "node view" and the front page, but you may also want a template for Views, particular blocks etc.

Do you expect certain elements to "grow" over time, for example number of items in the menu? Sometimes the themer has to adjust the layout to allow for such changes in content. Do you prefer a fixed or fluid page width? What about mobile devices?

Which browsers should the theme support? This usually means how far in the past you want to go for IE, IE6 is rarely asked for these days. Should the website in IE look exactly like in modern browsers or are discrepancies allowed ? This usually happens when using advanced CSS, some properties can be recreated in IE using proprietary Microsoft code that you put in separate CSS files called within conditional comments, it all affects the cost.

Thank you for asking :)

One layered Photoshop file is fine. If there are different page layouts, group those elements for easy access. You can include an overlay layer for each group with explanations, flows and other notes.

Keeping the explanations and the graphics together will simplify the process.