Folding paper is one of those skills that us adults generally take for granted. But some children need to be taught every little step that is involved.
The photo shows strips of paper that i used to teach a child 2 types of folding: folding along a dotted line and folding an edge to another edge. Starting with smallish pieces of paper makes the task slightly more easy to master. Some people may think it more motivating or more interesting to have a project to make something in particular, but this will mean that an increased number of factors need to be thought about and more movements planned. Keep it simple.
Folding along a dotted line is actually more difficult because one has to hold the image of where the line is, in one's mind. And if there is an issue with spatial placement then this will be even more difficult. So to learn, one has to start to fold, and then visually check, and repeat this, at the same time using an ability to keep it straight. The final step is to run one's finger along the edge. Not away from the edge but right on the edge. So placing the index or middle finger so the edge is positioned in the middle of the finger, then exerting pressure as the finger is moved along the edge. Exerting pressure and moving in the right direction and staying on the edge can be tricky for some children.
Folding edge to edge also involves visual spatial skills: bending the paper over and lining it up along the other edge can be difficult for some children. Once that is achieved, then one hand needs to hold the edges in position while the other hand needs to use a finger to again exert pressure along the folded edge to create the fold.