For me i like to start with my dress form and drape exactly what i have in mind, making a pattern from there, though this does waste fabric sometimes if you aren't careful to leave seam allowance or decide to go in another direction with the design. But it does let you see your design come together and make style changes from the beginning of the process instead of in the middle. This is particular helpful if you are copying a design from a picture but don't have a pattern. I also have several simple vintage patterns that I love and have made many times that I use as a quick comparison to my newly drafted pattern to see if i need to tweak anything, you may not have a pattern to check against but you may have a swimsuit that fits you well that you can use to help.
Most people these days prefer Lycra/Spandex Swimwear Fabric- Its very stretchy, dries quickly and is forgiving if you make a little mistake in the sizing. The downside however is that spandex shows everything especially when wet and not everyone is a fan of skintight fabric.
For Vintage enthusiasts who do not like Spandex-
There are many options if you want to use what I call normal fabric. For vintage suits they would have used anything from wool, which i wouldn't recommend, to cotton which is much easier to work with and more comfortable. Normal fabrics do take longer to dry, some will fade faster due to chemicals in pools and some fabrics may sag when wet. So if you are a serious swimmer this is probably not the way to go. I suggest testing your fabric to see how it behaves when wet and also see how sheer it is you may need to add an underlining especially if its a light color. wash both your fabric and underlining fabric before you start! This is very important as your lining may shrink differently than your outer fabric. As far as fitting you would want to measure yourself and add some ease just as you would any other item of clothing, most of your ease will be needed in the hip area I would allow 1''-2'' through the hip for sitting ease and about 1/2'' at the waist, the bust will need little to no ease for the best support.
Some vintage suits that I have come across will have a silky nylon panty attached inside the shorts, but they are often cut out because they get worn/stretched and the elastic gets brittle. Many of the vintage Hawaiian play-suits have this type of panty built in. Briefs can be added to your suit or you can wear a spandex bottom/panty you already have. I have also seen swim briefs cut on the bias if using a non-stretch fabric, this helps give a better fit around the seat. Bias cut bottoms are usually attached under the skirted suits some will have a panty liner of some sort instead of the nylon panty like in the shorts, I have also seen some 1970s bikini bottoms cut this way as well. Some vintage suits will have no panty lining or brief added and they would have just worn their own panties under the suit especially under the shorts style bathers. The patterns below are available on Etsy from different shops to give you an idea of the styles of suits I have seen these details used in, though these details are not called for in these patterns they are just examples.
And there you have it, just a few ideas for the pinup sewers out there, good luck with all your summer projects!