You can easily procure the required supplies for installing liners from a local home and garden store and they won’t cost you too much.
You have to mark the spots upon the aluminium flashing stock where you need to cut out the wood pieces for the planters. Use the felt-tip marker for this purpose. You should not forget to mark the drainage hole and ensure that it is a bit smaller than the wood planter’s hole present. This is to prevent the wood from getting damaged by the action of excess watering.
Use the shears or a hole saw to carve the drain holes, keeping it smaller than the planter’s hole. By keeping the hole smaller, the collection of water upon wood will be minimised and this will make the planter last longer.
Determine where the sides of the liner will be and then clamp two holes on the sides of the aluminium flashing stock. The clamps will allow you to bend the aluminium to 90 degrees angle easily.
Next you have to seal the liner to prevent water leakage. Use blind rivets at the spots where aluminium overlaps the edges of the boards. As for drilling holes for the blind rivets, you can use another board to support and keep the holes aligned.
Now insert the liner and secure it with rivets to keep it affixed in its position firmly.