While the oven is pre-heating:

Locate specific guides for disassembling your GPU if these steps are inapplicable to your GPU.

Remove the screws or clips holding your fan and any duct-work to the video card.

Gently take the fan/duct-work off.

Remove any old heatsink compound from the chips (using a paper towel).

Place the screws in a safe place.

Remove plastic components. Plastic will melt in the oven, and produce toxic fumes.

Elevate GPU around the edges, never let the components touch any surface.