NOTE - For US & Canadian visitors, "Drawbar" refers to the trailer "Tongue" - apologies for any confusion.
Lay the two drawbar sections down the centre of the trailer parallel with the chassis rails with the long edge vertical. Take one section and slide the inside end around till the inside edge is resting against the inside upright of the spring hanger and tack weld in position.
Repeat with the second section and then take the protruding sections and work them gently until the inside edges are together, Where the two drawbar sections meet, edge butt them together, level the tops and tack weld.
At this point it is a good idea to double check that the drawbar is central and to do this take a couple of measurements from the same point on the drawbar diagonally back to a point on the chassis in a couple of positions and check that both measurements from each side match.
If not you will need to recheck the position of the spring hangers and the lengths of the drawbar sections.
If needed reposition the hangers or grind the drawbar to suit.
At this point you can weld up the drawbar in all the downhand and vertical positions. Do not under any circumstances weld across the horizontal faces on the drawbar where it intersects the front cross member as shown below.
NOTE - Correct welding of the tongue to the chassis is important to prevent tongue failure during normal use.
The trailer plans on the website have specific requirements for NOT welding the tongue across the top face of the tongue section where it intersects with the front chassis rail.
The reason for not welding the top face of the tongue and most importantly across the front of the angle gusset is that when steel is welded, the heat from welding process alters the crystalline structure of the steel and creates a "heat affected zone" or HAZ. This HAZ is generally weaker and prone to cracking depending on the circumstances the welded section is placed under.
The tongue does a lot of work while towing. Every little bump in the road and every turn you make transfers stress through the tongue and compresses, twists and stretches the tongue material constantly. The majority of this flexing occurs where the chassis and tongue are connected and is enhanced when the trailer is constantly overloaded or unbalanced. This repeated loading and unloading (cycling) of stresses on the tongue can create microscopic cracks within the grain structure of the tongue material especially around the HAZ.
Over time a microscopic crack can grow, and potentially reach a critical size, to a point where the tongue may suddenly fracture and ultimately fail.
Fitting an angle gusset over the tongue top face gives addition area for supporting the load placed on the tongue and reduces the potential for tongue failure.