Assuming you have N shares
and you want to give someone X% equity
then the number of shares to give them is N * X/(100-X)
you have 100,000 shares
you want to give someone 10% equity
then you'll give them 100,000 * 10/90 = 11,111 shares
After you've given them their shares, they'll have 11,1111 out of 111,111 total shares = 10% equity.
Easy once you know the formula!
When you do a funding round on SeedLegals the platform works this all out for you, automatically updating all the numbers for you as you add/remove investors, change the valuation, etc.
But, if you're, say, giving equity to a new team member outside of a funding round then you'll need to do that calculation yourself and then add that person and their shares in a new Historic Round (or an existing one, as needed).