Most minilabs will develop your film properly, but the only way to guarantee ideal results is to go to a professional lab. Even in pro labs, though, things can go wrong. As an insurance measure, try not to develop more than two or three rolls of film at once.
When you take slide film to be developed, the staff will ask what services you require. The lab can develop and mount your slides for you, or for more professional-looking results, you can mount your own slides and cover the background with a custom-made mask as described on Creating your own mask and 6. Snap the mount together. If you want to mask and mount your own slides, ask for the “develop only” option without machine mounting.
Professional labs ...