1)Have all you ingredients ready following your recipe, pre-crushed barley malt, hops, yeast and other spices it may calls for.
Recipes for partial mash brewing are very popular and can be found easily on the web.
The barley malt can be previously crushed or milled at your LHBS or, if you have a barley mill, you can mill it yourself few minutes before use.
This is the barley malt before milling:
This is a barley mill, the barley crusher mill:
And this is how the barley should look like after been milled:
2)Heat half of your batch size water to 155F. If your batch is 5gal, heat 2.5gals of water.
3)Fill a muslin bag with the milled barley malt. Use two bags if it gets too full, then add to your water and leave it for 30min. You should try to apply heat just enough to keep 155F during that time.
4)Turn the heat off and remove the bag, let it drip for few minutes to collect all the good stuff. You can also wash the bag a little with some hot water at 155F to 165F to extract the most of flavors and colors.
5)Add your malt extract, liquid or dry. Stir well until completely dissolved.
6)Add water until you reach your boil volume, turn the heat on and proceed just as a regular extract brewing, boiling the wort for 60min, adding hops and spices as required, cooling the wort and pitching the yeast.
The basic extract process can be found at the "How to brew" menu