Shallowest possible layer of highly aerated soil, preferably scattered low mounds or berms rather than a solid blanket that fills entire area around tree. Stay off the trunk. Water enough to keep tree roots below new level moist.

Just be aware that the tree roots will quickly grow up into the new soil, so only plant things that can cope with dry shade - no matter how much you water, the tree roots will take most of it. Try not to plant things that will need frequent moving or dividing - mostly perennials, annuals only in pots - since the tree roots will be hard to dig through in future years. If you really NEED to plant annuals, then plant them in pots that you can scatter/stage among the other plants or on the mulch. Watering them will be much easier then also.

over time.. no matter what you do .. a maple will kill everything under it..

they are super competitors.. with a shallow root system that will steal all available nutrients and water ...

if you want to garden under trees.. shade gardening... you have the wrong tree ... i can refer you to the hundreds of posts about maples in the hosta forum ...

i once .. on a mature tree.. added 2 feet of soil in a raised bed around a 50 foot tree ... within 3 months... you could NOT put a shovel through the added soil as the maple COMPLETELY invaded the new soil ... now i know better.. and it did not kill the tree ...

good luck
ken

Variety being discussed not as large-growing as Norway, Sugar or Silver maples and probably not quite in the same league. Small-growing maples are fine garden trees. Probably 'Warrenred' will be intermediate in size when mature.