Transmission repair is an advanced skill. Depending on the issue you may need special knowledge or tools to fix the transmission. Here are a few things you can check and fix on a broken transmission.
The easiest thing to notice is any clear physical damage to the transmission. Something like a large crack should be easy to spot. Be sure to check around the housing.>
Next you should clean the transmission, which will make it easier to spot any other problems. By cleaning it all moving parts in the transmission should move freely and without getting stuck or caught on another part. If you do not have the proper cleaning equipment you can take the parts to a self-serve car wash.
There are a lot of small parts that can be replaced easily. These parts include bearings, bushings, and clutches. If you need these parts there are kits available made just for transmission repair.
The seals are a different part that may require replacing. Make sure you buy the correct seals for your transmission. When you reassemble the transmission it is essential to take your time, and to make sure that the parts are correctly placed and aligned.
For more tips on transmission repair, see the linked video..