This can be done in many ways. Some people will say you need to start with a final gravity and work backwards. Final gravity can be affected by mash temperatures, grain bill (use of dextrins, etc), yeast attenuation, and some mallard reactions during the boil. So it gets you thinking, how dry do I want my final beer? This can be based on style or personal preference.
If you want a dryer beer you can add cane sugar, corn sugar, brown sugar- any simple sugar that any yeast strain can easily metabolize. The ratio or sugars will lean towards simple sugars. This is important because yeast strains have different attenuation. This means yeast with high attenuation will more easily metabolize complex sugars, and hence, eat away more of the sugar mass in the wort. While lower attenuating yeast have genes that make it harder to eat those complex sugars leaving a higher final gravity.
Next, mash temperatures will affect the enzymes that rip apart the starch found in the malt. Higher temperatures will be favorable for enzymes that produce dextrins and some of the more complex sugars. Depending on the yeast, there could be more of those complex sugars left after fermentation and add to the residual sweetness. The dextrins are never metabolized by the yeast and serves as less of a sweetness and more as mouthfeel. So, if you want nice mouthfeel and a dry finish: mash at high temps and get a high attenuating yeast strain (preferably to style). If you want mouthfeel and residual sugars use a high mash temp and a low-medium attenuating yeast. Of course, a dry finish can be obtained by a low mash temp (148-150) and to add mouth feel you can add malt like cara-pils.
Next think about the necessary grains that you want. If you were to brew a stout or a porter there are dark grains that will affect final gravity and mash ph. There needs to be a balance between how much specialty malts in you grain bill to the final gravity you desire. Programs like beer smith helps with this a lot.
Next you can think about alcohol you want in the beer. This can be determined by base grains in addition to the specialty grains determined previously.
Finally, depending on boil, you can increase the malliard reactions with a more vicarious boil which will affect the final gravity. You could remove a portion of the wort and boil it separately that will lower the water volume and increase melanoidins. (used in scotch ales, as example)
The hops you can use based on style. Really just decided what flavor you want based on the region the beer style originated. Do a bitter hopping, flavor hopping and if necessary one or more different aroma hopping schedules. (knockout, hopback, and/or dry hop)
Make a nice yeast starter right before you brew and aerate your wort. Control temps. After the first trial is complete submit it into competitions and get in-biased opinions, but also make sure you decided what you would want to change to improve the beer. Make the beer again only changing one thing (the main "problem") you determined was holding it back. Keep doing this and you will find out how everything is affected by small individual changes in the process. Eventually, you will get a beer you are happy and proud of, and you will have gained a ton of information for future recipes and hopefully it will become more intuitive to design future beers.
Cheers!