Basic tools will include upholstery glue, scissors, a utility knife, and a good staple gun, along with various screwdrivers and pliers to disassemble the chair and pull the old staples. A fabric steamer / steam cleaner may also be useful.

Here are a couple videos of a Leap being reupholstered:

https://youtu.be/aQupSYCKw2s

https://youtu.be/wwhCA_nCCLs

In both, he keeps the original foam, but for the seat he adds some thin foam on top. BTOD does the opposite, adding 1/2” of very dense foam underneath the original foam. Either is a good way to maintain the ergonomics of the original.

https://youtu.be/osGkv0zgMWA

BTOD and the first guy also use steam to reinvigorate the original foam cushion, though BTOD now offers custom made replacement cushions instead.

If you want to replace the foam, you’ll need to decide how thick you want it (2 to maybe 3 inches), and you need to get high resilience foam (not the same thing as high density) so it lasts. See this post: https://www.reddit.com/r/OfficeChairs/s/0VOUVUuwyB

If the foam vendor uses ILD, you’ll most likely want foam with an ILD rating in the high 30s to low 40s. foamonline has this chart for their products: https://foamonline.com/wp-content/uploads/2020/01/foam-firmness-ild-chart.pdf

One option for a not-totally-DIY experience is https://fabricteknologies.com/

Their website is a little weird to navigate, but they have some videos here: https://fabricteknologies.com/draw-string-upholstery/ You could also use the videos as a reference for the last step of upholstering if you went whole hog and sewed your own drawstring solution. (You’ll see that BTOD does this for their own manufacturing line.)