You will need to reduce the tar down by simmering it next to the fire. It doesn't take a lot of heat to get this stuff bubbling so don't get it too close to the fire otherwise it will set on fire, if this does happen don't worry, just move the can away from the embers and it should go out again

Obviously you can't touch the can whilst it is that hot so i find a stick with a small split in the end is a very handy tool. Push the stick onto the rim of the can so that rim is held in the middle of the split.
Carry on reducing the tar down. Dip the end of a small stick into the tar every so often and let it cool, you can test how tacky the tar is with your fingers. It might take another few hours until the tar begins to set on the end of the dip stick. When the tar does start to set take the can away from the fire and let it cool for a while. You can now tell for sure whether it sets hard enough.

You can scrape out the tar from the can while it is still warm and gooey, and then mould it onto the end of a stick. When you need to use some of the tar simply hold your tar loaded stick over a flame for a second or two until it goes gooey again and then apply it quickly to the surfaces you want to stick. It sets very fast so you've got to be quick.

Have fun!
More bushcraft articles by Jon, just like this one at www.bushcraft.ridgeonnet.com

Also have a look at these other fantastic articles by Jon:

Make your own Comb 

Carved Dish 

Kuska Cup

Beech Bark container

Berry Picker 

Fish Trap