The most important thing on a weld is to make sure there are no inclusions in the finished job.
So:
1. Flush all mud and rubbish out and allow to dry;
2. Get in with a grinder, Dremmel or weapon of choice to clean the rust off the failed material;
3. Protect bare metal with weld through primer;
4. Jack vehicle to release stress from side rail as far as possible to allow chassis to take factory shape.
5. Ensure that chassis rail is straight, use clamps, jacks, chains anything that won't melt;
(5a). Stop drill ends of crack at this point, but do not weld holes closed, many sectors of industry no longer bother so it's a personal choice really;
6. Grind uniform 'vee' into chassis rail along crack as weld prep, do not go deeper than 1/3rd of the material thickness;
7. Stitch weld crack (20 x 20) starting from the middle then alternating either side allowing to cool between runs;
8. Clean area, remove clamps, drink tea etc;
9. Complete weld run starting from the middle again and allowing to cool often;
10. Clean weld with wire brush and protect (paint outside & waxoil inside)
11. Take photos to refer to if crack reappears, and the reason for not welding the stop drills closed is so you can see if the crack is still going or changes direction after the repair.
Remember: safety first, the author implies no guarantee and good luck.