Once you have a pattern piece that you think might work, wrap it around the doll, the way the clothing will go and see if it fits. It also helps to have the unclothed doll on the paper so that you know how long the clothes need to be. It's always surprising to me when I see how wide the pattern pieces need to be. Then add in seam allowances and enough width to allow the clothes to move easily on the doll, as well as putting the clothes on the doll. This gives you a starting point for how wide pattern pieces for sleeves and pant legs need to be. Be sure you include the measurement around the arms and legs. Use a tape measure on your doll and take a thorough set of measurements for what ever body part you are making clothes for.
I tend to be a free hand designer so having the lines there simplifies the process for me. I like having the paper on my cutting table because the inch markings are already on the table and it's very easy to see the lines through the paper, so I know my lines are straight when I need them to be. If this method works for you, great, but I find it is too fragile and tends to tear to easily. Some designers like to use tissue paper, like commercial patterns are made of. The easiest way I've found is to lay a few sheets of computer or copier paper out on my cutting table. When I set out to create a new outfit for one of my dolls, creating the pattern comes first. If you are a beginning doll tailor, I recommend using the patterns that are available here at the Doll Making Site, or buy commercial patterns, until you are more familiar with doll clothing construction. It helps if you already have a good understanding of what clothing patterns look like.
Here is the method I use when designing a new pattern for doll clothes.