What you want to achieve is something like this:
- Start by creating the CT spawn
- Connect the CT spawn to the two bombsites which should be on opposite ends of the map. One of the bombsites should be closer to the CT Spawn (the player should be able to get to that BS choke point in 3 to 5 seconds), the other a bit further way (less than 10 seconds to get to the choke points). Please also remember that CTs will use this path (BSA-CTSpawn-BSB) to fast rotate between bomb sites after the bomb is planted, so the total time it takes to get from BSA to BSB trough the CT spawn should be less then 15 seconds, this is because in competitive play the bomb explodes 35 seconds after it was planted (although the default time is 45 seconds) and it takes 5 or 10 seconds, depending on the player having a defuse kit or not, to defuse the bomb. So if the CTs get to the BS 15 seconds after the bomb was planted they have another 10 to 15 seconds to do the retake.
- After that create a path that connects both bomb sites directly. This path is very important for both teams. Before the bomb is planted Ts will use this path to fake which bombsite they are attacking. After the bomb is planted this is the path that the CTs will use to safely try to retake the bombsite and defuse the bomb. It should take longer to rotate between bombsites using this path than it takes using the CT spawn path (mentioned in the previous point), so something around 20 seconds, which leaves 5 to 1o seconds left to do the retake.
Many maps also limit the direction of movement in this path, that is they usually make it faster to move from BSA to BSB, than from BSB to BSA for example. That happens because one bomb site will always be easier to retake than the other, so by limiting the direction of the movement you can keep the timings equally fair for retakes on both bombsites.
For example in Dust 2 if you come from BSA through the stairs you can jump from the balcony into the middle ramp and use z-halls to get to bomsite b. if you're coming in the other direction you take almost 5 seconds more because you can't climb into the balcony, you've to go around it.
- It's very important to understand the job of these two paths. The path that connects BSA-CTS-BSB should be the fastest way to rotate between bombsites but also the less secure, that means that this path should be as open as possible and sniper friendly (but not too friendly).
The path that connects BSA and BSB directly should make the rotations take longer but also be more secure, that means lots of cover and narrow paths that favor the use of subs and pistols. This is a typical challenge/reward scenario, the harder the challenge the better the reward should be.
- After that add the T spawn and two paths that connect it directly to both bomb sites. Both teams should get to the bombsites ate the same time, but you don't have to be precise. If there are one or two seconds of difference then team that gets there last should have a clear view of the bombsite entrance in order to be able to slow down the other team using flasbangs.
The last thing to do for the Ts is to connect these two paths to the middle section (the one that connects BS A and BS B directly), this allows them to better use split tactics and do fakes. One of these connections should be open in order to favor snipers, and the other more closed in order to favor mid/close range combat. You can also connect the more open path to the CT spawn to add some early sniper action and more flanking tactics.
- Moving on to the bomb sites the BS closer to the CT spawn (BS A) should have 3 possible entrances, one which comes directly from the CT Spawn, one that comes directly from the T spawn and a third one that that allows the Ts to flank the CTs. This is the the bombsite that is usually defended by 3 CTs, and in which the Ts use more elaborate tactics, like 3-2 spits and stuff like that. The third entrance also usually favors gun and pistol fights, that means that it has lots of cover that make snipers useless.
The other bomb site (BS B) should only have 2 entrances, one that comes directly from the CT spawn, and another that comes from the T spawn. This is the bombsite that is usually used for rush tactics, and defended by just one or two CTs, since there is only one way in for both teams.
The last thing to remember is to add "safe areas" for both teams before each bomb site entrance. These are areas in which the players can take a few seconds to rest, think or wait for their team mates before they attack enemy, and usually also provide good defensive positions for the defending team.
- Finally it's also important that the bomb sites are placed at a higher level than the spawns. Besides aiding in navigation (go up to get to the bombsites, down to get to the team spawns) this also helps the defending team (which usually just have to wait for the other team ti take action, putting them at a disadvantage) giving them a better view of the battle field, and better aim since they are on a higher ground.
Edit: just noticed that the bomb sites on the picture are miss labeled, were it says BS A it should say BS B, and where it says BS B it should say BS A.