Rain gardens are best set 10 feet away from a foundation, so your downspout needs to be extended past the walkway at the least. You could tunnel under the walkway with pipe, or cut out a section of the walkway to hold a trench drain to carry the runoff.

A rain garden is a wide low spot that collects water until it drains into the ground. Most rain gardens are 150 square feet, which provides maximum runoff retention and room for the best assortment of plants. You should do a percolation test where you want to put the rain garden by digging a hole about 6" wide and 12" deep. Fill the hole with water and allow the water to sink in completely, then refill the hole and time how long it takes to drain a second time. If it takes longer than 4 hours you have poor drainage.

To create the rain garden a bowl shape is dug out and partially refilled with a good garden soil with plenty of compost. The depth of soil removed can be as little as 12", though heavy, poor draining clay may need twice that depth. The excess soil is piled into a low berm on the downhill side of the excavation. The added soil should be an inch or two lower than the surrounding ground. You can also add compost to the edges of the excavation and the berm soil. Place large gravel in a fan shape where the downspout or channeled water enters the rain garden to prevent heavy runoff from digging out soil. You may also need to place gravel or rocks where runoff from heavy or prolonged rain flows out of the garden. Ideally this outflow would spread out over lawn. You can surround the rain garden with bricks or stone if needed.

Do a search for "rain garden plants for your area" to find lists. If you don't find a site for your state you can use one from a nearby state with a similar climate. Rain garden plants can range from perennials to small trees, but most plants in these lists prefer sun. If your garden is shaded, look for plants that do well in shade. The plants can usually stand a short inundation, but will also do well during dry periods. Plant those that prefer more moisture in the low spot and those that will take drier conditions around the edges or on the berm. As with any garden bed, place plants that grow taller at the back and shorter plants at the front from where you would view them. Initially, plantings will look sparse, so cover bare soil with one to two inches of mulch. Consider placing a stepping stone or two in the middle of the garden so you can get in to weed without compressing the soil. Be sure to check the rain garden for overflows so you can prevent erosion and keep the flow where you want it. Over time some plants may not do well, so replace them with proven plants. Below are some photos of rain gardens.

Very Shady Rain Garden

Runoff From Street

Bordered Rain Garden