A Question About Poser

I've been teaching myself 3D art for a bit now, with a little help from others, but I'm stuck with trying to make patterns as textures - like adding stripes around boots or gloves - I can never seem to get them to join up properly, or curve properly, and they turn out looking messy - is there some trick I'm missing with it?
I've tried using both MS Paint and Photoshop, but both give the same results.
I can open texture files and alter colors, add stand alone symbols as well as bump maps - but stripes that curve with the object and join together properly just doesn't work - are there any Poser artists here who might be able to tell me what I'm doing wrong?

I don't use Poser, but it sounds like a uvmapping issue. The poser model is formed out of polygons. In order to texture a 3d object, the polygons are laid out on a 2d surface in what's called a uv map. The uv map designates how the 2d textures are projected back on the 3d object.

In order to unwrap a 3d model into this 2d template, the geometry is flattened and seams where it connects on the model are split so that the uvs can be made flat for texturing. Probably, when you are making these patterns you are jumping across uv seams. If you can get a picture of your model's uvs, you can plan around the seams/distortions. Otherwise it will be trial and error.

The things I'm trying to add stripes too don't have template files - it's a pair of boots and a pair of gloves, and when the MAT files are "unwrapped" from the object, it's very hard to match the lines up so they join properly.
I can draw a straight line on paper, but not on a computer

OK. This is basically a topology exercise - in 3D space, lines become curves, and vice versa.

Here's what I would do. Find the default MAT file for the boots/gloves, it should be in the same install directory. Make a copy. Then draw lines on the copy, first down the center, then to either side of the center line.

Then draw half ovals (bottom half) across that center line, start with semicircles and gradually flatten them from the top. You can use the circle tool for this.

Then save the scratch file (or export/flatten it if you're using Photoshop) and apply the new MAT to the boots in Poser.

Find the lines that most closely match "straight across" the boots, that'll be your reference.

Go back into Photoshop and erase all the semicircles that don't match, and then create more of the ones that do. Adjust as necessary.

Once you have a boot that has a 'grid' drawn on it, save that as your template file in PS, with the lines on a layer you can hide before you export/flatten to jpg.

Do the same with the glove MAT file.