You will need the shins you will be adding length to, cardboard or plastic pattern/support for the extension, plastic weld, sandpaper, primer, and paint.

Form the plastic/cardboard support to match the curve of the bottom of the shin and secure in place. Mark on it where you will be adding the plastic weld to.

Mix the plastic weld and begin adding it in layers until you have built it up to ~1/4" thick along the bottom edge. Make sure you have a good coverage along the bottom of the original shin to ensure it bonds properly.

Note: Mix the plastic weld in small amounts and apply quickly because it dries very quickly. In large quantities, it heats up too much and turns yellow. When cured, it forms a flexible plastic.

Trim back the plastic to the edge of your pattern.

Begin sanding the plastic weld extension, working from files to finer grades of sandpaper. The plastic will come off in small curls as you sand it rather than in a powder. Keep sanding until there is no longer a noticeable ridge between the shin and what you have added to it. The best way to gauge this is by touch. Some fine scratches can be filled with primer.

Prime the shins and then paint.