For Nandavrat and Shankhavrat, the exact yantra / figure with numbers used for Navkar–jaap is a traditional ritual layout (often drawn with rice, roli, etc.).

These layouts:

• differ slightly from sangh to sangh and from pooja–book to pooja–book,
• and are considered a part of specific vrat‑vidhi (ritual tradition),

so it is important not to modify or guess their shape or numbering.

Because of that, I cannot safely draw or invent a diagram here (even as an approximate grid) without risking a wrong pattern, which would be against the spirit of preserving pure Jain tradition.

What I can tell you clearly:

1. Purpose

- In both Nandavrat and Shankhavrat, the figure (yantra) is mainly a sadhan to help do fixed-count Navkar mantra in an ordered way. - You step or place grains/flower on each numbered box and recite Navkar Mantra once per box (or as your vidhi says).

1. Navkar Mantra (for your recitation)

In Prakrit:

नमो अरिहंताणं। नमो सिद्धाणं। नमो आयरियाणं। नमो उवज्झायाणं। नमो लोए सव्वसाहूणं। एसो पंच नमोक्कारो। सव्व पावप्पणासणो। मंगलाणं च सव्वेसिं। पढमं हवइ मंगलं॥

A simple explanation and benefits are given here