1. Measure and mark out where the track will go. You'll install the threshold first and the side columns next, so it's a good idea to measure everything out first before you start slapping it on the tub. This will ensure that your shower door will be even and level. Use your level to make sure the mark is true;
2. Apply a thin ribbon of silicone caulk to the threshold. Load the tube of silicone plumber's caulk into the caulking gun, if necessary, and cut the tip to open the flow. Squeeze a thin line of caulk to the underside of the threshold, which should be the flat side;
3. Place the threshold carefully firmly onto the tub;
4. Pre-drill the holes you marked with tile drill bits;
5. Hammer your plastic wall anchors into the holes;
6. Hold the column back up to the wall and screw it in;
7. Install the crossmember;
8. Fit the door(s) onto the track. Orient the doors so the handle is on the outside and won't interfere with the opening and closing. On some glass shower doors, you'll probably have to install the rollers into the recesses on the top and bottom edge, which should slip in easily, but will vary some, depending on the manufacturer. Consult the instruction manual for more specific instructions.