I have only granulated gelatin in small bags. Recipe is calling for gelatin sheets. How do I convert the gelatin I have into a gelatin sheet

Gelatin sheets come in different strengths so any conversion will be a guess.
You could try converting at a ratio of 3.5 sheets to a packet of Knox.
Or go by the rule:
One packet (1/4 oz.) should set 2 cups of liquid firmly or softly set 3 cups of liquid. Hope that helps.

4 sheets of gelatin is approx .25 oz package of powdered gelatin. This is approximately 1 Tbsp of powder