On my 1958 TR3A, I use a dark brown goop (Aviation #3 - I think it's called) that I apply to one face of the cork seal and a thin layer along the inner edges or lip of the valve cover. Then I put the seal into the lip around the valve cover and leave it overnight on a clean flat surface (like a table-top) and the day after, I set the cork a bit better and put the valve cover onto the head and tighten it down a bit. The next day, I tighten it down all the way.
For the next few days of running, I check it and if I see a bit of oil leaking, I tighten it down a bit more and wipe up the bit of oil. At this point, it's usually sealed. If I find a few days later that there is still a bit of a leak, I remove the valve cover and re-adjust the cork gasket. It is rare if you have to go through all this.
My original cork gasket was sealed this way when I removed it the first time at about 40,000 miles from new. During that time, I would guess that I had removed the valve cover and reinstalled it using the original cork gasket about 8 or 10 times by then when I would check and re-gap the valve clearances.
After this, it never leaks. I can take off the valve cover easily, many times over, because the cork is bonded only to the inside lip of the valve cover and not to the head. I can remove the valve cover when I seal the cork gasket this way for about 35,000 miles or more.
BTW, this past May 23rd, we celebrated my 50 years of ownership of "TRusty".