Par cook the vegies or meat you want in the pakora. I usually do a mixed variety of vegies: spinach, brocolli, potatos, a little onion.... If you're doing a mix, keep the vegies small or keep them large enough to do as individual battered items. Some chopped cilantro is also a good addition to small or large items.
As to the batter, I mix it by eye.
Some chickpea flour (besan)
water about 1/2 to 3/4 by volume as besan
a little oil
ground cumin or coriander, not both, you want a distinct spice flavor, not a blend, well at least that's my experience, Somewhere around a teaspoon per cup of besan.
red pepper flakes or powder to your heat preference
some baking powder. This is optional, I just like the lighter fritter that results
Mix this together. Your final batter texture depends on what you're battering. For a fritter of mixed small chunks, a thicker batter is needed. For a batter covering individual large chunks, a thinner batter is better. Whole raw leaves of spinach work too.
Heat the oil for frying to 350-375 Fry away. Timing varies by size and batter thickness. Pakora of small mixed vegies take longer for the batter to cook through to the middle.
Shrimp or seafood I wouldn't par cook, just batter and fry. Same for paneer cubes.
Serve with a some different chutneys. Tamarind, green chile, mint are all good.