You will first have to determine the size of the raft that you plan to make in order to settle the number of empty oil drums that you will require. For a simple one-person raft, one or two drums will suffice. However, you will need more drums if you plan to build a bigger raft.
When you have the required number of drums at your disposal, thoroughly check all of them to ensure there are no holes in any of the drums. Remember that the drums will be in contact with water most of the time, so you have to prevent them from rusting. Apply a fairly heavy coat of rust-free enamel to all of the drums. Allow approximately 24 hours for the enamel to dry properly.
After the enamel has dried, lay the drums horizontally side by side. The drums need to be held together tightly and for this reason you will have to use a piece of rope long enough wrap around all the drums. Secure the rope by tying a square knot.
There will be gaps at the points where the drums meet each other. Place wooden poles at these points along the length of the drums.
To further secure the drums and to prevent the wooden poles from sliding off the drums, you will have to lash together all the wooden poles with another piece of rope. Doing this will add more durability in the being-built raft.
Finally, you need to test the raft to ensure that it is perfectly safe. Move it into shallow water and leave it there to get wet for at least a couple of hours. Get the raft out of the water and retighten one or all of the ropes if required.