So, I have Hex Workshop, Mesen Emulator, Dan's stake, and a rom for CV. If I wanted to start creating levels from scratch or add screens, how would I go about it? I don't have intention of just changing backgrounds or sprites.
Open Stake and load your ROM (I highly recommend the PRG 1 version), you will see the very first screen of the first level, push the 5th icon, that one with the numbers and the cross (TSA editor Ctrl + T), now a new window will appear with all the background tiles, left click one of them and then left click on the square (meta-tile) that you want edit, those on the vertical column to the right, right click on said meta-tile to swap between color palettes when needed.
Right click one meta-tile of the main canvas to ''copy'' the selected meta-tile and then ''paste'' with left click.
At the bottom of the program, click the cyan arrows to see the previous/next screen.
Some 8x8 tiles have a ''floor'' value, so be careful, you may ended up falling or getting stuck not placed correctly.
A ''stairs'' pointer must be added too his meta-tile to work, click the ''New'' button above the columns on the right, an icon will appear, relocate and right click the icon to change direction of the stairs to match the meta-tile (up, down, left , right)
Next to the ''New'' button of the stairs, there's the breakable wall button, do the same, relocate and right click to change the item to drop (health, money bag, Double/Triple, etc).
You can also change several things with this tool, like colors of the backgrounds, place enemies, damage modifier, swap stage music, etc.
If you wanna import new graphics to the game (both tiles and sprites), use YY-CHR, just keep in mind the tiles with the ''floor'' value, sprites are a little bit more tricky at the beginning but turn out easy once you get it.
Ah, screens, my worst enemy (jk), unfortunately, I don't know how to do that, is it possible? yes, but I haven't seen any hack ROM featuring such thing besides The Holy Relics and Cadence of Agony, whose are heavily modified in terms of gameplay, making them (almost) new/entirely different games. If you have seen any other hack ROM with modded screens, try to ask to the creator.