A Lower Strut brace or bar is a metal brace between the two lower points of the front suspension, running under the engine. It's relatively easy to install and improves handling.
Raise the front wheels of the car and position it on jack stands, ensuring it is stable.
You can remove the wheels for visibility, but you don't need to.
The Wishbones are the curved metal pieces connecting the bottom par of the strut to the chassis. They look like a lower case 'r'.
Remove the long bolts connecting the wishbones to the car chassis with a 19mm socket. You may need a breaker bar.
Don't worry about the wishbones moving or falling. They're 'cradled' in place by the chassis.
Simply place the ends of the strut brace over the holes you just removed the bolts from and re-insert the bolts. Add some Loctite to the bolt's thread to ensure they don't move.
Tighten each side in turn evenly, but do not fully tighten yet. You may need some force to 'tug' the strut brace into place.
Once the strut brace is in place ensure the bolts in firmly but not too tightly. You will need to lower the car before finishing tightening them up so that the rubber bushes within the wishbone are in they're 'natural' state with the car's weight on them.
With the car lowered and resting on it's own weight tighten up the bolts.
This can be a little tricky, so move use the steering wheel to move the wheels out of your way as best you can.