I know of three decent techniques.
One is pretty risky, expensive, messy, but requires little effort (providing you have a good wither skull farm). Making the perimeter is extremely easy. It's the cleanup afterwards that may create some problems.
In short - you spawn a lot of withers under the bedrock, then you afk for a long time above the bedrock until all blocks are cleared. Then the hard part comes - you manually kill all the withers. Video about the process:
The other approach is to dig down to the lava lakes level manually, then build the bottom part of a world-eater - just the sweepers part, with a simple attachment that replaces the entire upper part moving the stations and sweepers a block down on player's request - and launch it manually after lowering the "land" level by 1, to sweep up all the remaining lava.
The third approach is very expensive in terms of iron (3 ingots per 1 source) but if you have a very good iron farm, it's not a problem. Quaff a fire resistance potion, fill your inventory with empty buckets, and hold rightclick at the lava lake. Since lava buckets are unstackable, 15 out of 16 buckets will land in lava. Tweakeroo is very helpful providing replacement of depleted stacks of buckets in your hotbar, and Itemscroller helps crafting bulk amounts of buckets. It's the fastest way of getting rid of lots and lots of lava "manually", but expect a couple shulkerboxes of iron blocks to go into lava.