There are four major considerations in bottle slumping: cleaning, placing, firing, results.
A major consideration in slumping bottles is the cleaning required. This requires a lot of time, as everything has to be clean inside and out.
Labels and the glue attaching them must be cleaned off. The interior needs to be clean. And the bottle needs to be dry before being placed in the kiln.
Soaking first helps the cleaning process. If you are doing large numbers you will need to find large containers that you can leave the bottles in to soak for a couple of days. For small numbers, a few bottles soaking in a bucket of soapy water will do. The bottles should be upright to allow the internal residues to float to the top. After a few days the labels should have fallen off and the internal deposits floated to the top or easily washed out. Screen or plug the sink to be able to remove the residues from the sink before it gets into the drain. Then you can proceed to rinse and clean the bottles.
The bottles will need to be wiped free of the glues used to stick the labels to the bottles. Any glue residue left on the bottle will show up on the finished piece, usually as devitrification. If the glue or label has not come off with the cold water and soap soak, you can soak them in very hot water. You can then use a variety of solvents to remove the most persistent adhesives.
For the difficult internal deposits, you can use one or two short lengths of small chain and slosh that around with water. The chain used for hanging small stained glass panels is ideal.
As you can see the cleaning process is lengthy and can be time consuming. So you might want to see if you can get new bottles at a reasonable price. Home brew shops may have bottles they are willing to dispose of. Bottling plants may also have supplies of bottles. New bottles will greatly reduce the labour of bottle slumping, although it does not fit the re-cycling ethos that brings people to bottle slumping in the first place.