Punch holes first is good advice.
Cut out inside as required next.
After installing gasket trim the excess from exterior.
It is a good practice to cut the blank large so you will not be punching or cutting near an edge.
Thickness of gasket stock does not seem to matter to those who sell gasket sets..
They measure all over the place when checked with micrometer.
Gasket material at auto parts stores seems often to kept "in back."
Ask the clerk to bring out what they have.
Usually they have more than one thickness on hand.
I have had no luck finding suitable material to make heat resistant gaskets,.
Those I have to order as NOS.
Heavy duty paper bags are about the right thickness for water pump to gear housing gasket
used on Early Kiekhafer outboards. Grease the paper before installing.
An antique outboard may have leather seals which also can be made with punch and a little
imagination. Save leather scrap for a new purpose.
The $5 punch sets are good values for the money.
Punch will cut gasket cleanly if you use with end grain cutting block.
Maple is wood of choice. Other wood will work. Important thing is end grain that lets
knife edge of punch to go clear thru the gasket into the wood.
Louis
.