Making curtain meshes lowpoly

Share your files, please.

Well, it looks pretty low poly, already but there are other things you can do.

*Don't use a decimated mesh for your LOD 0. Decimate will murder your topology and create issues. Start with a high-poly mesh and dissolve edges (X > dissolve edges) you don't need.

*Before dissolving edges, select your mesh, tab into edit mode, and press alt + J to turn your tris to quads (this means fewer edges in your way). In the little window in the lower lefthand corner put a checkmark next to "compare UVs." This will preserve your UV layout. Sometimes turning tris to quads messes up the UV map.

*If you encounter a face that isn't in quad form select the face and press F. 

*Dissolve some edges in each of your plane meshes. There aren't any folds to preserve in those meshes so you can dissolve a lot of edges in them.

*In Plane.001 focus a lot on the top edges. There is a lot of unnecessary geometry there. Depending on your needs, you could just delete some of those faces altogether.

*You can delete geometry via the UV editor. Enable UV sync selection first. This way your UV islands will be visible and you won't have to select anything in the 3D window.

*The curtains that have backsides also have rims that aren't needed. Select and delete the backsides and the rims. Because of the topology, I can't find any quick way to do this so selecting these faces will be tedious.

Edit: One thing you can try. Select a mesh that has a rim, tab into edit mode, press A, in the UV editor select UV > seams from islands. Your edges will be red though the result won't be perfect. Then go to edge > mark sharp. Apply an edge split modifier and be sure to uncheck edge angle. Your seams will be split now. Select a face on the rim, ctrl + L, X > faces. Do the same selection and delete process on the backsides. This won't be perfect and won't work on every mesh because of topology problems. 

*Round part 1.003 has a lot of unnecessary edges at the top and bottom.

*Round part 1.002 can be reduced further.

*Dissolve any edge that isn't needed to create a fold. If an edge is not helping get rid of it.

*Delete anything that won't be seen. Anything faces that are covered up by another mesh. 

*If you need to add an edge loop. Do so with ctrl + R.

*Once you've deleted the backsides and rims, and dissolved unneeded edges, try to straighten the edges. Press G twice and your movements will be constrained to an edge.

*After this, select the meshes that need backsides and add a solidify modifier. Uncheck "fill" and set the thickness to whatever you need to keep the backsides from clipping into the front sides.

Last edit: the more I look at your meshes the more problems I find. Some vertices need to be removed. Select a mesh, press A, mesh > clean up > delete loose. Remove doubles with M > by distance (select your mesh first.) You will need to dissolve vertices that are left (X > dissolve vertices). You will need to merge other vertices (M). Again, if you have a high poly version start with that and manually remove edges as I explained above. This is actually quicker to do than cleaning up a messy mesh. 

Thank you Fwecka (Lolabellesims) (Lolabellesims) for answering  but that wasn't exactly what I was looking for. I was going more after the actual technique with wich I could make EA-kind curtains, lowpoly and retaining all the details. My file was merely an attempt, not the finished result. I hope I didn't sound too ungrateful because I really appreciate your answer!

If you do indeed want to preserve the folds, then you have to retopologize the mesh with the edge flow in mind. You just need to place edge loops strategically

They retopologize everything by hand, you can tell by the edge flow. Zremesher is not suitable for making low poly gaming assets, no automatic retopology tool is. It is a tool in a sculpting program and quickly turning sculpted objects made of millions of faces into something unwrap-able/editable/render-able is the only time it is useful. If an object needs to move or morph while being of reasonable polycount, it is pretty much useless

Everything looks great, as intended