My quick release rattles so how can I convert back to a fixed nut front axle?
This is achievable, but you're using a big hammer to fix a potentially small problem.
Simply removing the QR Skewer and slapping two nuts on the outside is a recipe for disaster. There is not enough thread going outside the fork's dropouts to hold a nut. There's 100mm (4 inches) of axle inside the wheel, and a QR only lets 2-3mm (not a lot) poke out either side. You need at least as many threads out each side as there are threads in your new wheel nuts.
So you need a complete new front axle, 2x cone nuts, and 2x wheel nuts for the outside. Its worth replacing the bearings at this time too, because they're cheap and may as well.
There are some slightly-custom tools needed too - picture shows a pair of thin cone spanners, which are only about 2mm thick. A normal spanner won't fit on the cone nut flats to hold it while adjusting the bearings.
If your axle looks like this then its a Through Axle ("thru axle") and you're on a fairly modern bike. Reverting to nutted wheel axle will require adapters that are custom to your frame/through axle design, and there are too many variations to explain here.
In a typical front wheel, whether for road or mountain bike wheel, one simply removes the locknut, washer and cone nut from one side of the wheel and slip the hollow QR axle out through the other side. Now would be a good time to remove the bearings (sometimes loose, sometimes held in a retainer ring) clean and regrease and replace the bearings and dust covers.
A typical solid axle for a front wheel is about 140 mm in length. This will give you about 20 mm on each side past the locknut's as most adult bike front wheels have 100 mm spacing (there's 100 mm of space between the front fork's dropouts and therefore the Over Locknut Distance--O.L.D.--is 100 mm). The extra length on a solid axle will be taken up by the width of the drop out (around 5 mm) and the width of the securing nut and washer outside the drop out.
To prep the solid axle for placement, determine where the outside edge of a lock nut should end up on the solid axle. If the spacing of your dropouts is 100 mm, subtract that from the total length of the solid axle. Divide this number by two and this result is the distance from one end of the axle that the Locknut for that side ends up. Because solid axles are a slightly narrower diameter than QR hollow axles, the cone nuts and lock nuts from your old axle cannot just be transfered over to the new, solid axle. So it's a good idea to get that hardware along with the axle. Oftentimes they are sold together as in the above link.
Place on one side of the axle the cone nut, washer and lock nut. At the point determined above, stop threading the lock nut, spin the washer and cone nut back to it and secure this side. Next, put the axle through the hub, spin on the other sides's cone nut, washer and lock nut. On a solid axle, the correct bearing setting is the point where all play is removed and the axle spins freely.
Place the wheel in the drop outs and secure with a washer and bolt outside the drop outs. To summerize, you need only replace the hollow axle with a solid axle and it's appropriate hardware.