In this guide, I'll show you how to easily install a FTB modpack. (For other modpacks, it's the same thing, only the launcher changes). I'll install the FTB Presents SkyFactory modpack. You can download the FTB launcher here. 

STEPS: 

1. From your control panel, click on the "Maintenance" and "Reinstall" menu to open the auto packs installer tool. Make a search       for the modpack.

2. Click on the wanted version and install the modpack. 

3. Now we need to start the server, go to the "Settings" menu. 

4. Check if the server version is present like this "ftb-presents-skyfactory-25.1.7.10.2.5.2". (If not, please do the step 2 again). 

5. Enable java 8 for recent modpack otherwise the server will not start. 

6. After each changement you need to click on "SAVE CHANGES". 

7. Finally start the server. 

8. Open the FTB launcher from your computer, install it if you didn't previously do so. Select the same modpack than the server           from the "FTB MODPACKS" menu or "3RD PARTY PACKS", it depends which modpack you have installed.  

9. Here it's important, you need to select the exact sub-version of the modpack. In the server I have the 2.5.2v, so in client side I         need to install the 2.5.2v too, otherwise you'll have the mod rejections issue during the connection. 

10. Click on the "Launch" button at the right bottom corner, it will download and install the modpack into your computer. 

11. Once the launcher is ready, open your multiplayer menu and add your server address to connect. 

If everything is fine, you'll see the green check symbol under the slot number, that means you can connect without problem.