Inside Electrum wallet software, click menu View and make sure you have enabled "Coins" tab.
On this tab, select outputs that you would like to spend. Select one/s that has closest value to what you want to spend. In case you need to spend exact value, select output with a higher value than what you need to spend.
Selecting is done using mouse right click on the output. Once you are done, click "Send" tab.
Fill in the "Pay to" field.
Click "Max" button to pre-fill maximum value to spend (sum of yours selected output/s). You may decrease the amount if needed.

Feedback is welcome.