theres no definite instruction on how to make a DIY ramp since it is do it YOURSELF, you're just gonna have to find the right materials, meaning wood is usually a bad idea since it warps, absorbs water and can harbor different types of growth due to moisture, you need to take measurements of the area in which you want to place your basking area in or on, make sure it is stable from any movement and uneven weight distribution and figure out what to use as a stable ramp as well as how to attach it safely to the basking area
i myself found a cheap plastic basket in my storage and cut out one side and used plastic ties to secure a coated bakers cooling rack to the edge and bent it so it dipped down into the water, i also have a large flat stone placed inside the basket to give it some weight so when my turtle climbs up the rack the basket doesnt flip over
this is basically the cooling rack i used and i bent it somewhere in the middle by wedging it somewhere tight and jumping on it till it bent, and i used plastic ties to secure one side to the basket
