Just a basic ball peen hammer works best like pictured below, the body hammer with pointed end is too pointed, it will pull/draw the gasket material inward, tried one of those many, many years ago. 8-to-12-ounce hammers work good with the ball end (rounded/domed end) with a more prominent/pointed end. Seems newer hammers in store are blunter. On larger bolts it's best to tap around the hole numerous places to make the initial cut, then once the gasket material fibers are basically cut, you can center the hammer on the hole take another hammer and with a few light taps finish cutting the material.
When making gaskets for the transmission case, that little tab will be in the bolt hole, pull material to the side and take a small screwdriver or shop pick push down on one side and it should turn so you can get ahold of it and pull it out. Lay material back down and run a bolt in to hold it in place, then go to the next hole, and so on. Get all of the bolt holes cut first and insert bolts.
On a piece like that with a straight edge on one side I use the factory edge of the material even with it to help keep it aligned. Next, start tapping on the inside of the case on about a 45º angle, you'll want to tap right on the sharp edge of the case. I'd advise starting on the side that has some material hanging over the edge, it will help keep the material from pulling inward. Once you go an inch or so, you can turn the hammer over and use the flat end and make faster progress until you get close to a corner. I'm assuming the corners are somewhat rounded so switch back to the peen end. Cut/tap around the corner, go another inch or so, then back to the flat end until almost to the next corner then switch back.
You don't need to hit hard, just some light to medium taps to start with until you see what it takes to cut the material. On a long run you may tap an inch or so, then tap back to make the clean cut, or flip the hammer and use the other side to make the clean cut.
Do know you'll have some fuzzies on the edges. For these, take a sharp box cutter knife and rake backwards on the edges. You'll want to hold the gasket close to where you're scraping and only go a few inches at a time. You may still have a few hanging on what was the lower side, just adjust the angle on the knife to wipe them off.
If there are alignment dowels, tap it with the flat end to mark it, then tap it at a 45º angle as most have a slight beveled end. Tapping on a 45º will cut it below the bevel if there is any. You should be able to push the material down over the dowel and continue. Just be careful pulling it back off to clean up the fuzzies.
It may sound a whole lot more complicated than it really is but isn't. It would be nice if you had a smaller piece to experiment with and gain some feel and confidence. But just take your time and you'll get it.
If you have any 8-12 oz. ball peen hammers in your toolbox, they'll probably work fine. I looked up new hammers at several big box stores, and Ace Hardware and they all seem to have pretty blunt peen ends. All of mine have come from Flea Markets and yard sales and have never paid over $3 ea. for them. Got to say I was surprised when looking up new ones. I've probably have 25-30 of those hammers with the correct points on them but still pick them up when I see them for a decent price. Having 4 shops, and several tractors with toolboxes they have a tendency to migrate off so I try to keep a good stock to save hunting one up.
I'm the same way with 1/2" and 9/16" wrenches. If it's made in U.S.A and $2- $3 it's mine. Never have enough as I may have 2-3 things I'm working on at the same time and usually leave the tools with what I'm working on so as not to have to chase them down. I hit the jackpot 2 years ago on our way to Johnstown Swappers Day stopping at a yard sale we stop at every year. 4 older gents get together and are selling off/downsizing their shops. Had this box of wrenches for $1 ea. I had half of them in my hand, and the guy looked at me and said $10 take the whole box which included the elusive 10mm wrench. DEAL..!! Got the Vise-Grip welding pliers for $5. It was a good day.
