The other responses you received are correct. You DO NOT need a screwdriver to push the center of the pin down, just push hard with your finger and it will pop-up.
After that, you will need to remove the plastic piece holding the OEM horn closest to the centerline of the vehicle by removing the two gray screws on the top and the one black screw from the front. Once you remove the 3 screws you will need to pull the plastic piece towards the grill to pop it out then pull up to remove. This will give you access to remove the horn.
Remove the yugo horns and install the new horns in the same place. Some people purchased extra wiring because the OEM wiring is not long enough, but all I did was remove/cut the electrical tape from a couple areas on the wire harness to give myself enough slack. You also have to disconnect the wire harness from the support rods Toyota hooked them up to by removing/cutting the electrical tape. This should give you enough slack to connect the wires to the new horns from the top. Test horns. If all is good, secure the horn/wire connection with electrical tape, retape the areas of the wire harness you cut earlier, and retape the wire harness to the support rods. Reinstall the plastic piece and engine cover.
When you reinstall the engine cover the push pins need to be at max height otherwise they won't engage/lock correctly (what I did was use the engine cover to push down on the bottom of the pin so it pops all the way up, this takes some effort). When you reinsert the locking pin, only push the pin down until it is flush with it's top. This will lock it in. If you push too far down it will unlock the pin and you'll have to repeat.
Trial and error, but it worked for me. Hope this helps.