Creating a Mood Board
The creative process starts with a theme. As a designer, you should have no problem creating an idea after completing your fashion forecasting. Once you have decided upon a theme for your design, you will create your mood board, which is also called an inspiration board.
What is a Mood Board?
Once you have decided upon your theme, you will transfer it onto a board, which will convey your feelings, emotions, and the style of your theme.
Collect visual references, as inspiration for your mood board like magazines, colour swatches, fabrics, or various other visual references.You will mount these inspiration pieces to a hard board, which you will then give a title, much like a book or film.
Do plenty of research by attending galleries, watching films or music videos, and visiting the latest fashion shows.
Take pictures of objects and anything related to your theme that relates to how the product makes you feel and how it will look. Make sure your mood board is not cluttered or messy.
There are two approaches to creating and delivering your theme ideas. 
Physical Approach
Digital Approach
Once you have chosen your approach, it is time to assemble your theme and bring your collection to life. Regardless of the avenue you have selected, the process will be relatively similar.
The Design Process
After you have laid out your design theme on your mood board, you are ready to begin design development. When developing a design, there are four steps that you will cycle through in the development process:
Colour Stories
A colour story refers to a theme or line of colours, which identifies each piece in your collection.
In the world of fashion, colours fall into the following four groups:
Staple Colours
Seasonal Colours
Fashion Colours
Accent Colours
Range Development
A range plan includes an overview of your collection, which includes your entire design and includes all of the financial parameters.
A typical range plan will usually include some or all of the following information:

The total number of garments you will be designing
Flat drawings or illustrations of each of your fashions
Proportions of each garment
Specifics on each clothing style
Different colour stories and fabrics for each garment
The cost and selling point for each garment
Order quantities for each style
Manufacturer details
Sizes

Fabric and Trim Selection
Fabric and trim selections play a significant role in the design process. Consider the following when you are making your choices:

Think about the current fashion trends.
Make sure each trim and fabric work for the particular garment you are assigning it to.
Always consider budget and production when making fabric selections.

Design Consideration
When it comes to garment design, there are some things that you as the designer should consider:

The needs of your customer and target market
The image you are representing
Fashion trends
The visual appeal and aesthetics of a garment
Functionality of your garment
The availability of materials
Resource and budget limitations

Illustrations
After all the research, brainstorming, and preparation, you begin the sketching phase.
Begin with a Croquis
A croquis represents a basic model drawing of pose, which you can trace over repeatedly. When selecting your croquis, remember to choose a body type that you would like to design for.
Sketch Your Original Design
After you have drawn your croquis, you should trace several copies in a variety of different poses. You can now sketch your original fashion designs, which can include one garment or an entire collection.
Boldly Illustrate Your Design
Illustrate the textures in your fabrics and emphasise the way you intend for the fabric to hang on your model.
Add Folds, Wrinkles, and Pleats
Emphasise the different elements in your design, which may include pleats, hemlines, and folds.
Illustrate Prints and Patterns
Finalise Your Drawing with Ink, Shading, and Colour
Sketching with Flat Schematics
In the fashion industry, you will work with three types of fashion drawing, which include the following:

Fashion illustration – Sketching or drawing on a croquis for a general fashion design project
Fashion float – A type of fashion illustration that is used for presentation boards or apparel line sheets without a model.
Fashion flat – A garment spec drawing or a fashion technical drawing

It is crucial that all your illustrations remain true to the garment, fabric, and the figure. When you follow these rules, it will save costly mistakes in the manufacturing phase. 
Croquis for Flats
Should be true to actual human anatomy and should contain technical lines on a fashion figure.
Principles of Flat Schematics
There are three fundamental principles that you should keep in mind when you are sketching technical drawings.
First Principle - Be accurate. 
Second Principle - Ensure your flat is communicating something visually. 
Third Principle - Technical drawings must be ‘flat’, as they are depicting the garment in a flattened state, and not how it looks on the body.
Garment Sample Stages
One important stage that is important in the fashion design process is garment samples. Garment samples have the following stages:
Muslin – First concept sample, which is usually sewn on an inexpensive fabric. 
Fit sample – To ensure the desired fit of your garment.
Sew-by sample – To estimate costs, this sample reflects all of the construction information necessary to produce the garment.
Sales sample – A sales sample moves production costs along and improves the quality of assembly.
Photo sample – Made to scale for your model for photography purposes. You can skip this sample.
Size run – A full series of samples made in every size.
Pre-production – The last garment sample is a final run through with all the details in place.