To create illustrations using Adobe Photoshop and Adobe Illustrator you need a scanner, 
digital tablet, and of course the programs.
First of all you draw a design, this could be a women dressed. You scan this draw. Then import this design from the scanner to the program Photoshop.
To clean your hand draw design from the scanner to the software you must do:
Menu – Image – Adjustment – Levels.
With that you must adjust the level black and white of the draw to clean the image.
It is convenient to scan your illustration in color mode then you must change to gray scale, that is Menu – Image – Mode – Grayscale.
So you have the illustration in black and white.
Go to the layers and include a new layer above which color must be white so that when you erase the illustration to delimit the lines of the draw, you can clean it totally.
Then the illustration is clean and ready to be colored.
To add color you must create layers with each part of the draw.
For example: the skin color is in a layer which is relevant to be name.
The hair also must have its own layer with the name hair.
And so on.
Go to the original design layer and defined the lines.
Use the tool bucket to color the image.
You can scan a fabric or import from the internet.
You must reduce noise, that is important to have a perfect fabric to use for create the texture and pattern you prefer.
You can bring to the illustration with a brush with low opacity and low flow shadows which do the effect more realistic.
The fabric can be scanned, created by digital and handmade done; inclusive it can be selected from internet.
You choose define pattern and will be kept for other illustrations.
To create a fabric on a jacket or trousers you must select the area individually and then choose de fabric, copy and paste into the selected area. Selected with polygonal lace the area using then tool transform, can rotate or deform (warp). This is very useful when you want that persist the pattern of the fabric perfectly.
Adobe Photoshop is more complete for illustrations because it is made on map of bits which define more a picture.
Adobe Illustrator is about vectors those are use for example for making a pattern of a garment. The lines of Illustrator doesn’t deform as could be in the case of Photoshop.
So for a nice complete illustration use both Photoshop and Illustrator.
Also for digital collage.
You can prepare a digital collage with images obtain on magazines, scanned or obtain throw internet. For that you must be careful about the resolution as we said before Photoshop can change the resolution because of the map of bits. Instead Adobe Illustrator is based on vectors so does not change the result.
The images can be cut for the collage with the tool Polygonal Lace for cut each piece of the collage.
So Photoshop can be used with the combination with Adobe Illustrator.
If you obtain images from magazines scanned them and apply filters as Gaussian Blur, or Unsharp Mask to obtain high quality of print.
Use Illustrator and import all the layers of Photoshop, use Convert Photoshop layers to objects. Those layers must be copy. With tool of selection of illustrator select, rotate, etc.
The collage has be assembly by Illustrator then with Photoshop do the postproduction.
The background could be draw by hand and scanned. Each element is copy and pasted in different layers.
Keep your archive on psd format in that way you can keep all the layers independent.
