Making cosplay armour can be quite inexpensive and still get good results with the right techniques. Not every costume will be practical due to the real-life limitations of materials, but most can be done with the correct attention to detail. Scrounging for pieces of armour and costume that work for a cosplay is common enough, but some pieces are unique enough to require being made from scratch.
Make a pattern of the armour from muslin, newspaper or poster board. Larger pieces may be easier to pattern with fun foam. This pattern is just the base of the piece and will not include any raised patterns or decoration.
Adjust the pattern until it fits correctly. Make the correct pattern completely from fun foam.
Cut a piece of styrene with about an inch larger on all sides for each fun foam piece of the armour.
Use the contact adhesive to attach the fun foam pattern to the styrene. When putting the pieces together, be sure to attach them so that they form the correct curvature of the piece. Pieces with a lot of curve to them may be easier to put together with hot glue rather than contact adhesive.
Draw the patterns for any raised decoration directly on the styrene. Use this as a pattern to properly cut out pieces of styrene for the decoration.
Hot glue the raised pieces onto the armour surface. Do not use contact adhesive in case the pieces need to be adjusted into place.
Spray paint the armour. Use masking tape to cover sections that should not be the colour being sprayed.
If the armour cannot be worn by itself, attach it to the costume below with thread the same colour as the armour surface. These stitches should be tiny, so they will not be very noticeable.
Make all the garments that go under the armour before the armour itself. The measurements of the armour may need to be adjusted to fit your measurements while wearing the rest of the costume. Actual historical armour can sometimes make good base references for cosplay of characters without very many good reference pictures. Unusually round pieces will need to be specially made using other techniques. There is a limit to how much flex fun foam and styrene can properly hold. Samurai armour used rice lacing to hold pieces together. This appears on some anime and video game character armour. The same effect can be achieved with shoe laces.
Be sure to follow the instructions on the contact adhesive. Most require a few minutes of sitting time before being used. Contact adhesive does not allow for much adjustment when it is attached. Be sure everything is properly lined up before sticking it together.