Every Home Has Problems.
If you are shopping for a new house, or a new-to-you house, there will be something not quite right. New homes settle, have nails that will pop out, and doors that could have been sanded a bit better before painting.
Old homes have problems like mine.
My home's Achilles heel is the bathtub. It settled wrong. Toward the room side of the tub it is flush against the tiles. Toward the back side of the tub there is a gap of as much as a half of an inch. More than a Centimeter for the Imperial Measurement Impaired.
I did this exact repair a little more than a year ago. I was improvising and had a case of Right Church, Wrong Pew. When I did that, I believe that I used the wrong materiel. In other words, the vinyl tubing that I used as a Backer Rod did not stick well to the silicon tub caulk. Over the year it pulled away, water got in, and it "popped" loose.
My own job worked well but not knowing that someone else thought it through before me created this little time bomb.
The way you install it is fairly simple.
Clean the crevasse of dirt, grime, mold, old caulk, and whatever water may have gotten in there. It must be absolutely dry. Caulk will not stick (well) to old caulk.
Begin by pinching the Backer Rod down between your fingers and slide it into the crack that you need to fill. Push the Backer Rod in the crack using a tool. While they do have special tools for this, I found myself using my fingers to do most of the work, then a screwdriver to make sure the rod was firmly in place. The Backer Rod must be snug against the wall and the tub so you will need to select a Backer Rod that is thicker than the largest gap in your repair job.
The surface of the Backer Rod must be behind the surface of the wall facing you. You do not want the Backer Rod to protrude past the surface of the tiles that are being worked with.
Continue packing the Backer Rod into the crevasse until you have filled the crack down to what you consider a normal gap for caulking. I used two different thicknesses of Backer Rod to make sure that I filled the entire gap, firmly. I didn't want to repeat this in Spring of 2015.
Consider that your caulking job will be "neater" if you use masking tape to cover the area of the tiles that are near where you caulk. This way you won't have to use a utility blade or knife to score your "overcaulked" area so that you may have a "nice sharp line" of caulk on the tub.
Once you have installed the Backer Rod, then caulk the area as usual. Make sure that there are no gaps left after the caulking has completed. The Backer Rod should grip the wet caulk but it does have a different surface and therefore will hold the caulk differently.
Of course with any home improvement task, your mileage may vary. Just take your time. Measure twice and cut once.
...and most importantly:
You can do it yourself. If I could do this you certainly could!