I do my string quilts on cheap copy paper. Sometimes I draw lines, other times I just randomly place strings. Strings can be all same size or different sizes (widths). If you do use paper as a foundation, decrease your stitch length to facilitate paper removal. I "lock stitch" (stay stitch) at the beginning and end of seams. This way the stitching doesn't pull out when you remove the paper. (I only do a stitch or two which I find is enough to lock the seams). Do not remove the papers until the quilt top is complete. I have heard of some people using muslin for the foundation and not using a batting, just a backing, for a lightweight summer quilt. There really are no rules when it comes to strip quilts. Some folks go with a certain colorway, others go completely scrappy. Some choose a single color for one element of the quilt (see above post by zozee - her #1 item) to be used in the same place on each block. I am just starting a string quilt for QOV which will be red, white and blue. The only consistent elements will be the white fabric and a flag fabric in one corner. (I am doing a variation on Sunshine and Shadows or Roman Stripe. The white will be the 1/2 of the block, with red and blue stripes in the other 1/2. The flag fabric will be in the corner opposite the stripes. Doing this will form an alternate block within the top. Will try and post the first 4 finished blocks later today on this thread).
There are no hard and fast rules. String quilts are fun to make and are all very attractive. They are a great way to use up scraps (although my scraps seem to multiply exponentially!) Good luck and let's see what you come up with.