The first thing to consider is where you're going to do the installation. If the outside temperature is  higher than 75 F the installation should be done out of the direct sun light. The adhesive on the Velcro requires 24 hours to take full bond and during that period extremely high or low temperatures should be avoided.

Next we want to make sure the areas where the Velcro strips are going to be placed are clean and dry. The best way to do that is with a clean rag and rubbing alcohol.
Now you're ready to install your headliners.

Cold weather installation instructions:

It is important to work in an area that is clean and free from dirt. The adhesive should be applied at room temperature, with the ideal room temperatures being 65°F to 75°F, and relative humidity 40% to 65%. (The lower the humidity, the better the bond.) Excessive cold, heat, dirt and relative humidity will be detrimental to your adhesive performance. If adhesive must be applied at temperatures lower than 65°F, warm the substrate and the tape to elevate temperature prior to applying. This can be done with a blow drier or electric heater and will help the adhesive cure.

Please read this paragraph if you live in climates that drop below zero degrees F.:

Even though the Velcro adhesive is temperature rated for -20 degrees F we don't recommend its use in climates that drop below zero. 

Removing your headliners:
Start in a corner were you can get a finger in under the panel. You can also carfully use a flat screw driver or putty knife to get a corner down enough to get your fingers in. Gently pull down with one hand while slidding the other hand up under the panel. What you want to do is slide one hand between the two sides of the Velcro to seperate them. Just start at one side of the panel and work your way across in this way until all the Velcro is seperated. You'll find that the panels will come down quite easily like this, but to just try to grab a corner and pull them down is almost impossible. 

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