First of all, you will need to have a clean fire. Make sure that there is no clinker in your fire pot and that the tuyere is clear to allow air to flow up through the fire.
The first suggestion that I would have is to try forge welding a single piece, NOT a "drop the tongs" type of weld. It is much easier to focus on the technique of welding without having the awkward problems of aligning the pieces before they cool too much. Simply take a scrap piece of bar stock, cut HALFWAY through and fold the end back over itself (kind of like a tight "V"), and try with that. You also want to make sure that you heat both sides thoroughly, wire brush all slag away, apply plently of flux, and get the piece up to a good white heat (just about where it starts to sparkle and burn). Once there, remove from the fire and quickly get it to the anvil and tap the 2 halves together. It doesn't take a lot of force. Forge welding DOES NOT have to be pounded together, you just have to make the surfaces come in contact. As a matter of reference, I saw a demo where a smith showed us that you can forge weld a bar using a wooden mallet. Not a lot of force. If it doesn't stick the first time, put it back in the fire, heat, wire brush, and try again.
Also, ALWAYS think safety. Wear the correct protective gear, and be aware that forge welding typically involves flying sparks/slag.
Hope this helps,