I will summarize the rendering process of the classic Build engine:
1. Collect walls into "bunches". A bunch is a group of neighboring walls that face you.
2. Sort bunches. Sorting is a brute-force order n^2 process, but n is low, thanks to the "bunch" concept.
3. Rotate and project vertices to screen view, clipping to the left or right edges of the screen if necessary.
4. Rasterization/Scan conversion. The top and bottom edges of each wall for each column is calculated.
5. Wall clipping. Clips each column independently, using Build's top/bottom vertical span buffer. For transparent walls (red lines in the Build editor), add the walls of the sector behind it and repeat from step 1 (after texture mapping).
6. Texture mapping. Uses highly optimized assembler routines (for 486/Pentium).
Ceilings and floors (that are not sloped) are drawn with horizontal spans for speed purposes. Everything else is drawn in the vertical direction.
How do you make slopes like those in the Build?
Your question is too general. Slopes are just an extension of non-sloped ceilings and floors.