Mongoose model d 70 r. series 1, how do you tighten the bearings on the pedal crank

The crankset and pedals on that bike or any other are not unique to one model. Either the original or compatible replacements are available at a variety of sources, but there are several dimensions that are important. Best to go to a bike shop or bike co-op to get info on what is needed and available.

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If I remember right the threads on the pedals are left handed, which means you turn in opposite direction than normal. then you unscrew the nut on the crank. The race for the bearings will have a slot. Use a screwdriver to turn it the normal way. Take out that bearing. Then move crank a little way to the other side to get bearing out from the inside race. Then you should be able to pull crank through.
On the newer bikes, the crank can be dismantled on each side so all you have is a shaft to remove. You still remove the race and bearings the same way.

Usually takes special tools. The pedals should be removed, the crankset is either a single arm or a 3 piece unit. If its a single, remove large nut on one side of the crank, usually the left side. Then remove the inside cone, the bearings are visible and should be removed. The last is the crankarm itself. It will only come out if thew left side bear is removed. Twist it gently and it usually comes out at the 3 o'clock position.
If its a 3 piece crankset, special tools are need to remove. SAuggest you take it to the nearest bike shop.

On the right hand side of the crank or oppisite side of the front sprocket should be a 1 1/8 lock nut remove lock nut an washer should be an ajustable race with two holes known as a spanner race tighten up the race reinstall washer an lock nut tourqe down lock nut rotate crank making sure bearings dont drag an done

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Almost all Mongoose models use the standard BSA 68.
I would write a question to them at:
http://mongoosebikes.net/mongoose-bmx-bike/20-mongoose-mode-270-v2-freestyle-bmx-bike/
This bearing should not fail during the normal life of the bicycle, unless the rider weight is over 80 Kg.

If your pedals are not tight DO Not ride, until you have tightened them. You will strip your crank arms and have to replace them. Not even counting that you can get seriously injured. You need a pedal wrench to tighten the pedals, But, I have no idea if your crank bearing and crank axle is okay, I highly doubt it. I would take it in for service to a bike shop, it is kind of like someone trying to fix their own air conditioning , they make more work for you. And as a person who has been in the bike business for 38+ years, I am telling you straight, you need someone who has experience and the right tools.

Could have sticky or seized up crank bearings where the pedals are or the rear wheel bearings. Have you tried pedaling and spraying a lubricant where all the bearings are or remove and re-grease the bearings?

Dry or bad bearings in either the right pedal or the bottom bracket (shaft between the pedals).

See this article, particularly the paragraph, "Symptoms of a bottom bracket needing overhaul"...

http://en.wikibooks.org/wiki/Bicycles/Maintenance_and_Repair/Bottom_Brackets/Bottom_bracket_overhaul

take the pedal off the opposite side that the chain is on the there should be two bolts turn the first one towards the back tire and take the washer out and use a flat head screwdriver to tae the second ne out the same way pull the first set of bearings out then pull it through the frame it might be a little hard but it will come out make sure you take the chain of the sprocket then put everything on the same way

im certain that model has a 3 piece crank set up. the inner crank bearing jam nut has loosened up or the bearings themselves have gone bad.first check to see if its tight, if it is then it will require a replacement set of bearing that are a pretty standard size to all bikes and can be purchased at nearly every bike shop.