This job is plenty easy to do, you can do the job in place but it can make you create new words. We recommend you to remove the trailing arms from your car to do it perfectly. If you wan to make it in place, start by unbolt the cv joints from each side of the differential and the lower mount of the shock to drop the trailing arms to the maximum and give you enough space for the process.
1– Remove paint/coating/grease for where you gonna weld the new parts with a simple sandblaster or just a brush grinder.
2-Spot weld your parts in some corners to avoid it to move from the good position
2.1-Fitment can be required to get less gap as possible
3-Weld it. Mig welder will do the job perfectly. Tig welder need really more works to fill gaps and its not an obligation for a perfect penetration