Realistic Finger Cymbal (zills) sf/converting .wav to .sf?

Step 1. Obtain samples. There are many free/public domain samples. The sample MUST be in .wav format. If you find .mp3/.ogg/.flac/.aiff (etc.), and want to use those, you will have to convert them at some point.

Step 2. Edit the samples. This is easily done in Audacity ( a free program). Cut any silence that comes before the sample or after the sample. Adjust the amplification of the samples so that all samples are the same volume (on audacity, effects-> amplify). You can also do other things here if you choose, such as using a noise eliminator (be careful, this could affect the quality of the sample) or using an IR Reverb plugin in conjunction with Audacity to add reverb (all of these will be found under the effects tab, you will have to set up Audacity to work with your external plugins. VST effect plugins work fine.) You can export to .wav in this step.

Step 3. Load all samples into a soundfont editor (I will use Polyphone in this explanation). Set you root key to the note that you want to have you sample play on (in MIDI, 60=middle C). Loop you sample by clicking on a start point and end point and adjusting your loop until it is smooth (no clicks). The loop will allow the sample to play theoretically indefinably, so do not include the attack or decay in you loop (unless it is a percussion/piano/harpsichord, then loop at some point in the decay). Use the equalizer to limit unwanted noise or adjust the sound of your sample if needed.

Step 4. Create an "instrument" and name the instrument whatever you want. Load all of your samples into the instrument by selection them, right clicking and selecting "bind to instrument". In this stage, add the range you want your samples to play with the "key range" space, and what dynamic levels you want it to play at with the "velocity range" space. You will also have to adjust the "volume envelope release" a bit to make your sound font play smoothly. For the volume envelope step, I suggest you apply it to the "global" (this will adjust all samples).
If you use stereo samples, make sure they are properly panned (left=-50 right=50). There is a lot more you can do here, but since you are just starting, that is all I would worry about.

Step.5 Create a Preset and name it whatever you want. Add your instrument to the preset by selecting it, right clicking and selecting "bind to preset". After this you are done. Once again, there is a lot you can do in this menu, but I would not worry about it since you are just starting.

Save your soundfont and you are done.
EDIT: while this may seem simple, it actually takes a couple days to a couple months to complete this process (depending on how many presets and samples are involved) if you want a precise, clean and realistic soundfont. Just be aware when you start a project, it will not be a quick finish.